New package: Tcl v0.2.0

[JuliaRegistrator]

• Registering package: Tcl
• Repository: https://github.com/emmt/Tcl.jl
• Created by: @emmt
• Version: v0.2.0
• Commit: 99214047fe9159c0a9d8c7c366ab3c342524722e
• Reviewed by: @emmt
• Reference: emmt/Tcl.jl@9921404#commitcomment-177053913
• Description: Optimized Julia interface to Tcl/Tk
• Release notes:

# Breaking changes

- Instances of `TclObj` no longer have a type parameter as the *type* of a Tcl object reflects a cached internal state that may change for efficiency.

- `Tcl.getvalue(obj)` has been replaced by `convert(T, obj)` to get a value of given type `T` from Tcl object `obj`.

- List-specialized methods (`llength`, `lappend!`, and `lindex`) have been suppressed in favor of the Julia API for collections. Thus, `length`, `append!`, `push!`, `delete!`, `getindex`, and `setindex!` shall be used to access Tcl objects as if they are lists.

- `Tcl.getinterp()` has been replaced by `TclInterp()`.

- `Tcl.setresult` has been replaced by `Tcl.setresult!`.

- Tcl/Tk options `-key val` in scripts or commands are produced by `key => val` pairs in Julia code where `key` is a string, a symbol, or a Tcl object. Previously it was done for keywords in function calls but, then, `key` cannot be a reserved Julia keyword.

# Fixed

- Raw pointers in C calls are protected by having their owner object preserved from being garbage collected.

- Getting a single char from a Tcl object works for multi-byte sequences.

- To protect the content of shared objects, Tcl imposes a strict copy-on-write policy. Not following this policy causes Tcl to panic (i.e., abort the program). To avoid this, attempts to modify a shared Tcl object are detected and forbidden by throwing an exception.

# Changed

- By default, the package uses artifacts `Tcl_jll` and `Tk_jll`. To use other libraries, one can set the environment variables `ENV["JL_TLIBTCL"]` and `ENV["JL_TLIBTK"]` to the absolute paths of these libraries before calling `pkg> update Tcl` and `pkg> build Tcl`.

- `Tcl.doevents` has been deprecated and replaced by `Tcl.do_events` which returns the number of processed events.

- Following Tcl behavior, `obj[i]` yields `missing` if index `i` is out of range.

## Added

- `Tcl.do_one_event(flags)` to process any pending events matching `flags`.

- `TclInterp` constructor can yield the shared interpreted for the thread of the caller or a new private interpreter.

- `Tcl.quote_string(str)` yields a proper double-quoted string that can be inserted directly in Tcl scripts.

- A Tcl variable may be unset (or deleted) by meany different means (at the user convenience). For example: `Tcl.unset(interp, name)`, `Base.delete!(interp, name)`, or `interp[name] = unset` (where `unset` is the singleton provided by the `UnsetIndex` package and exported by the `Tcl` package).

[github-actions]

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[ViralBShah]

Package name is ok and I can override that. But we probably want to close this PR anyways, and fix a few things before opening a new one.

@emmt Is that ok?