Add more Unicode planes to regular_char

[stasm]

Let's extend the regular_char production to be more permissive of characters from outside of BMP. A good standard to follow is https://www.w3.org/TR/REC-xml/#NT-Char.

fluent/syntax/grammar.mjs

Lines 411 to 417 in 3c7cd30

	/* Any Unicode character from BMP excluding C0 control characters, space,
	* surrogate blocks and non-characters (U+FFFE, U+FFFF).
	* Cf. https://www.w3.org/TR/REC-xml/#NT-Char
	* TODO Add characters from other planes: U+10000 to U+10FFFF.
	*/
	let regular_char =
	charset("\u0021-\uD7FF\uE000-\uFFFD");

[stasm]

Extending the reference parser to support astral Unicode planes turned out to be easy thanks to Unicode-aware regexes in ES2015. I opened #179 with the proposed implementation.

[stasm]

The definition of NT-Char in the XML spec comes with the following note:

Note:

    Document authors are encouraged to avoid "compatibility characters",
    as defined in section 2.3 of [Unicode]. The characters defined in the
    following ranges are also discouraged. They are either control characters
    or permanently undefined Unicode characters:

    [#x7F-#x84], [#x86-#x9F], [#xFDD0-#xFDEF],
    [#x1FFFE-#x1FFFF], [#x2FFFE-#x2FFFF], [#x3FFFE-#x3FFFF],
    [#x4FFFE-#x4FFFF], [#x5FFFE-#x5FFFF], [#x6FFFE-#x6FFFF],
    [#x7FFFE-#x7FFFF], [#x8FFFE-#x8FFFF], [#x9FFFE-#x9FFFF],
    [#xAFFFE-#xAFFFF], [#xBFFFE-#xBFFFF], [#xCFFFE-#xCFFFF],
    [#xDFFFE-#xDFFFF], [#xEFFFE-#xEFFFF], [#xFFFFE-#xFFFFF],
    [#x10FFFE-#x10FFFF].

Should we include something similar in the Fluent spec?