fix parsing of long chars

[pfitzseb]

Fixes #.

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[StefanKarpinski]

This is actually a parser error in Julia's own parser:

julia> '\U10000001'
ERROR: syntax: invalid escape sequence

The largest valid code point is U+10FFFF.

[pfitzseb]

Yeah, that's basically what this test tests:

julia> CSTParser.parse(raw"'\U10000001'")
  1:12  errortoken( CSTParser.InvalidChar)
  1:12   CHAR: '\'

julia> CSTParser.parse(raw"'\U00000001'")
  1:12  CHAR: '\U00000001'

Could've chosen a less arbitrary codepoint though, I suppose.

[StefanKarpinski]

Wouldn't it make sense to test the two values on either side? I.e. test that '\U10FFFF' is valid and that '\U110000' is invalid?