feat: add %symbol% substituting to non-clone time ices like atload, etc

There are many such symbol var-like symbols:

- %ID%          – id-as ice
- %USER%        – username (in user/plugin ID)
- %PLUGIN%      – plugin name (in user/plugin ID)
- %URL%         – snippet url
- %DIR%         – plugin directory path
- %ZPFX%        – value of $ZPFX
- %OS%          – `$OSTYPE`
- %MACH%        – `$MACHTYPE`
- %CPU%         – `$CPUTYPE`
- %VENDOR%      – `$VENDOR`
- %HOST%        – `$HOST`
- %UID%         – `$UID` (numerical user id)
- %GID%         – `$GID` (group #)

With this patch many has been fixed (like %ID%, %USER%, %PLUGIN% were
returning empty strings, possibly breaking packages, where this undocumented
feature is used) and support for load-time ices (like `atload''`, etc.) have
been added.

For example:

```zsh
zinit id-as'plugin-%UID%' atload'print Loaded from dir: %DIR%, plugin id: %ID%' for zdharma-continuum/null
```

output is:

```
Loaded from dir: /home/q/.local/share/zinit/plugins/plugin-500, plugin id: plugin-500
```

README.md

@@ -675,6 +675,35 @@ Also, two articles on the Wiki present an example setup
 [here](https://zdharma-continuum.github.io/zinit/wiki/Example-Oh-My-Zsh-setup/).
 
 # How to Use<a name="how-to-use"></a>
+## Special variable-like strings `%NAME%`
+
+You can use a set of special var-like strings in any of the ices (currently: `atinit`, `atload`, `atclone`, `atpull`, `mv`, `cp`, `make`, `configure`, and a few others):
+
+- %ID%          – id-as ice
+- %USER%        – username (in user/plugin ID)
+- %PLUGIN%      – plugin name (in user/plugin ID)
+- %URL%         – snippet url
+- %DIR%         – plugin directory path 
+- %ZPFX%        – value of $ZPFX
+- %OS%          – `$OSTYPE`
+- %MACH%        – `$MACHTYPE`
+- %CPU%         – `$CPUTYPE`
+- %VENDOR%      – `$VENDOR`
+- %HOST%        – `$HOST`
+- %UID%         – `$UID` (numerical user id)
+- %GID%         – `$GID` (group #)
+
+For example:
+
+```zsh
+zinit id-as'plugin-%UID%' atload'print Loaded from dir: %DIR%, plugin id: %ID%' for zdharma-continuum/null
+```
+
+output is:
+
+```
+Loaded from dir: /home/q/.local/share/zinit/plugins/plugin-500, plugin id: plugin-500
+```
 
 ## Ice Modifiers<a name="ice-modifiers"></a>
 

zinit.zsh

@@ -1169,11 +1169,12 @@ builtin setopt noaliases
 
     local -A ___subst_map
     ___subst_map=(
-        "%ID%"   "${id_as_clean:-$id_as}"
-        "%USER%" "$user"
-        "%PLUGIN%" "${plugin:-$save_url}"
+        "%ID%"   "${${${id_as_clean:-$id_as}:-$___id_as}:-$ICE[id-as]}"
+        "%USER%" "$___user"
+        "%PLUGIN%" "${___plugin:-$save_url}"
         "%URL%" "${save_url:-${user:+$user/}$plugin}"
-        "%DIR%" "${local_path:-$local_dir${dirname:+/$dirname}}"
+        "%DIR%" "${${local_path:-$local_dir${dirname:+/$dirname}}:-$___pdir_path}"
+        '%ZPFX%' "$ZPFX"
         '$ZPFX' "$ZPFX"
         '${ZPFX}' "$ZPFX"
         '%OS%' "${OSTYPE%(-gnu|[0-9]##)}" '%MACH%' "$MACHTYPE" '%CPU%' "$CPUTYPE"
@@ -1198,11 +1199,14 @@ builtin setopt noaliases
     ___add=( "${ICE[param]:+${(@Q)${(@z)ZINIT[PARAM_SUBST]}}}" )
     (( ${#___add} % 2 == 0 )) && ___subst_map+=( "${___add[@]}" )
 
-    local ___var_name
-    for ___var_name; do
-        local ___value=${(P)___var_name}
-        ___value=${___value//(#m)(%[a-zA-Z0-9]##%|\$ZPFX|\$\{ZPFX\})/${___subst_map[$MATCH]}}
-        : ${(P)___var_name::=$___value}
+    local ___cnt ___var_name
+    [[ $1 = <-> ]] && {___cnt=$1; shift;} || ___cnt=1
+    repeat $___cnt; do
+        for ___var_name; do
+            local ___value=${(P)___var_name}
+            ___value=${___value//(#m)(%[a-zA-Z0-9]##%|\$ZPFX|\$\{ZPFX\})/${___subst_map[$MATCH]}}
+            : ${(P)___var_name::=$___value}
+        done
     done
 }
 # ]]]
@@ -1405,8 +1409,6 @@ builtin setopt noaliases
         .zinit-setup-params && local -x ${(Q)reply[@]}
     }
 
-    .zinit-pack-ice "$id_as" ""
-
     # Oh-My-Zsh, Prezto and manual shorthands.
     [[ $url = *(${(~kj.|.)${(Mk)ZINIT_1MAP:#OMZ*}}|robbyrussell*oh-my-zsh|ohmyzsh/ohmyzsh)* ]] && local ZSH="${ZINIT[SNIPPETS_DIR]}"
 
@@ -1416,6 +1418,12 @@ builtin setopt noaliases
     filename="${reply[-2]}" dirname="${reply[-2]}"
     local_dir="${reply[-3]}" exists=${reply[-1]}
 
+    # Substitute special strings 3 times deep, like %ID%, %USER%, %PLUGIN%, etc.
+    @zinit-substitute 3 id_as filename local_dir dirname 'ICE[atinit]' 'ICE[atload]'
+    [[ -f $local_dir${dirname:+/$dirname}/$filename ]] && exists=1
+
+    .zinit-pack-ice "$id_as" ""
+
     local -a arr
     local key
     reply=(
@@ -1609,9 +1617,15 @@ builtin setopt noaliases
     local ___mode="$3" ___limit="$4" ___rst=0 ___retval=0 ___key
     .zinit-any-to-user-plugin "$1" "$2"
     local ___user="${reply[-2]}" ___plugin="${reply[-1]}" ___id_as="${ICE[id-as]:-${reply[-2]}${${reply[-2]:#(%|/)*}:+/}${reply[-1]}}"
+
+
     local ___pdir_path="${${${(M)___user:#%}:+$___plugin}:-${ZINIT[PLUGINS_DIR]}/${___id_as//\//---}}"
-    local ___pdir_orig="$___pdir_path"
     ZINIT[CUR_USR]="$___user" ZINIT[CUR_PLUGIN]="$___plugin" ZINIT[CUR_USPL2]="$___id_as"
+
+    # Substitute special strings 3 times deep, like %ID%, %USER%, %PLUGIN%, etc.
+    @zinit-substitute 3 ___id_as 'ICE[id-as]' ___pdir_path 'ICE[atinit]' 'ICE[atload]'
+    local ___pdir_orig="$___pdir_path"
+
     if [[ -n ${ICE[teleid]} ]] {
         .zinit-any-to-user-plugin "${ICE[teleid]}"
         ___user="${reply[-2]}" ___plugin="${reply[-1]}"