Allow full unicode range

At a glance

• Identifier: #849
• Stage: RFC3: Accepted
• Champion: @leebyron
• PR: Allow full unicode range

Timeline

• Commit pushed: Editorial on 2022-06-02 by @leebyron
• Added to 2021-07-01 WG agenda
• Mentioned in 2021-07-01 WG notes
• Added to 2021-06-03 WG agenda
• Mentioned in 2021-06-03 WG notes
• Commit pushed: Revised RFC after feedback on 2021-05-18 by @leebyron
• Added to 2021-05-13 WG agenda
• Mentioned in 2021-05-13 WG notes
• Spec PR created on 2021-04-13 by leebyron
• Commit pushed: RFC: Allow full unicode range on 2021-04-13 by @leebyron

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This spec text implements #687 (full context and details there) and also introduces a new escape sequence.

Three distinct changes:

1. Change SourceCharacter to allow points above 0xFFFF, now to 0x10FFFF.
2. Allow surrogate pairs within StringValue. This handles illegal pairs with a parse error.
3. Introduce new syntax for full range code point EscapedUnicode. This syntax (\u{1F37A}) has been adopted by many other languages and I propose GraphQL adopt it as well.

(As a bonus, this removes the last instance of a regex in the lexer grammar!)

Reference implementation https://github.com/graphql/graphql-js/pull/3117

Closes #687 Closes #214