close #16646; since now works with bootstrap nim post csources_v1

[timotheecour]

• close {.since.} doesn't work for bootstrap nim, nor 1.0.0, breaking valid code #16646 (follows csources_v1 CIs: attempt to use csources_v1 #16282 and ci changes to use it throughout in refactor all code that builds csources #17815)
• revive promote math.isNaN #16627 (/cc @xflywind) now that csources_v1 was merged
• things like dedent can now be used without worrying about bootstrap (/cc @a-mr for things like WIP: use dedent for de-indentation in lexer, fix #15184 #16699 (comment), WIP: use dedent for de-indentation in lexer, fix #15184 #16699 could be revived if needed, see also fix RST parsing when no indent after enum.item (fix #17249) #17257 (comment))
• improve rendering of a rst warning:

Paragraph.

A. stringA
B. stringB
C. string1
string2

before PR:

/Users/timothee/git_clone/nim/timn/tests/nim/all/t12224.rst(6, 1) Warning: RST style:
  not enough indentation on line 6
      (should be at column 4 if it's a continuation of enum. list),
  or no blank line after line 5 (if it should be the next paragraph),
  or no escaping \ at the beginning of line 5
      (if lines 5..6 are a normal paragraph, not enum. list) [warnRstStyle]

after PR:

/Users/timothee/git_clone/nim/timn/tests/nim/all/t12224.rst(6, 1) Warning: RST style:
not enough indentation on line 6
  (should be at column 4 if it's a continuation of enum. list),
or no blank line after line 5 (if it should be the next paragraph),
or no escaping \ at the beginning of line 5
  (if lines 5..6 are a normal paragraph, not enum. list) [warnRstStyle]

(note that nim now has ascii unit separator to separate messages so there is no ambiguity here); this is now consistent with other messages which don't indent on the line right after the note eg:

/Users/timothee/git_clone/nim/timn/tests/nim/all/t12225.nim(6, 1) Error: undeclared identifier: 'bar3'
candidates (edit distance, scope distance); see '--spellSuggest':
 (1, 0): 'bar1' [proc declared in /Users/timothee/git_clone/nim/timn/tests/nim/all/t12225.nim(4, 6)]
 (1, 0): 'bar2' [proc declared in /Users/timothee/git_clone/nim/timn/tests/nim/all/t12225.nim(5, 6)]
  bar3()

future work

in future work, dedent itself could be improved though:

• it doesn't work at CT
• it (or indent) should have an optional pram to allow specifying indentation level after dedenting, so that this would be possible:

block:
  block:
    let a = """
    foo1
    foo2
    """.indent(2, fromMargin = true)

producing:

  foo1
  foo2

but note that in the case of this PR, this is note needed.