misc cleanups

[timotheecour]

• remove {.importc.} for js types
  Add more JS stuff to dom.nim nim-lang/Nim#13483 (comment)

[timotheecour]

• compiler/extccomp.nim should use json API instead of manually writing json (eg source of bugs like koch boot fails if even an empty config.nims is present in ~/.config/nims/ [devel regression] nim-lang/Nim#13633, and less readable etc)

[timotheecour]

• make exportc imply Used for modules, see fix #13218: avoid some irrelevant warnings for nim doc,rst2html,--app:lib, + other fixes nim-lang/Nim#13550 (comment)

[timotheecour]

• Fix gdb scripts nim-lang/Nim#13658 (comment)
  => use nim program tools/nimgdb.nim exec or portable version of it

[timotheecour]

• => remove old bootstrap symbols < 0.20.0 #389

[timotheecour]

deprecate isNil

see also timotheecour/D_vs_nim#36 (comment)

Edited:
nil == something doesn't work with C or JS backend sometime.

[timotheecour]

• importcpp > importjs for js

[timotheecour]

• replace

## some ``foo``
=>
## some `foo`

everywhere in nim files since it's simpler and compat w markdown

[timotheecour]

• cleanup $nim_prs_D/tests/vm/tvmops.nim

template forceConst(a: untyped): untyped =
  ## Force evaluation at CT, useful for example here:
  ## `callFoo(forceConst(getBar1()), getBar2())`
  ## instead of:
  ##  block:
  ##    const a = getBar1()
  ##    `callFoo(a, getBar2())`
  const ret = a
  ret

=> a.static

[timotheecour]

• add to NEP https://nim-lang.org/docs/nep1.html
  it's subjectVerb, not verbSubject, eg: deprecate existsFile for fileExists

=> nim-lang#14732

[timotheecour]

• => remove old bootstrap symbols < 0.20.0 #389

[timotheecour]

• proc paramStr*(i: int): TaintedString = raise newException(OSError, "paramStr is not implemented on Genode")
  => this should be a different kind of Exception, maybe doAssert false

[timotheecour]

• (=> misc cleanups #64 (comment)))

[timotheecour]

• remove all absolute html links and replace with relative links eg:

<https://nim-lang.org/docs/lib.html#pure-libraries-modules-for-js-backend>`_

[timotheecour]

• add test case for nim js: appending a uninitialized string appends a null char '\0' nim-lang/Nim#11788
  (just the part about:

this is related to nim-lang#11783 but seems different, as the bug in nim-lang#11783 was only present when both appended and appendee were uninitialized, which isn't the case here.

var ret = "foo1"
ret.add fun()
)