Support full unicode in parser

[dondonz]

This PR implements the RFC to support full Unicode in the parser.

Key spec changes

• GraphQL now supports a wider range of Unicode characters. SourceCharacter was expanded to include any Unicode code point that is neither a leading nor trailing surrogate. Previously only up to U+FFFF included
• Spec now includes guidance on Unicode surrogate pairs
• (minor) GraphQL now allows certain control characters

Key changes in this PR

This PR has two halves:

1. UnicodeUtil, used by StringValueParsing. This is to handle braced escapes and escaped surrogate pairs
2. ANTLR grammar changes, which expand the definition of a SourceCharacter to mean any Unicode code point that is neither a leading nor trailing surrogate

References

RFC GitHub issue: graphql/graphql-spec#687
RFC spec text: graphql/graphql-spec#849
RFC JS implementation: graphql/graphql-js#3117
Previous PR: #2335

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Want a Unicode fun fact? Groovy fails to compile if there are any Unicode code points that are not exactly four hex digits. You'll even encounter this compilation problem in COMMENTS.

For example: this comment containing RFC text will cause a compilation error

For example the input `"\uD83D\uDCA9"` is a valid {StringValue} which represents the same Unicode text as `"\u{1F4A9}"`.

The fix is to add an extra backslash

... which represents the same Unicode text as `"\\u{1F4A9}"`.