Use ES6 syntax for astral Unicode characters

[Seb35]

PR type

• Bug fix (non-breaking change which fixes an issue): no
• New feature (non-breaking change which adds functionality): yes
• Breaking change (fix or feature that would cause existing functionality to change): no
• Documentation change: no

Prerequisites

• I have read the CONTRIBUTING.md document: yes
• I have updated the documentation accordingly: no but it should be done indeed
• I have added tests to cover my changes: there is one

Description

This PR is constructed on #586 but uses the ES 6 syntax for astral Unicode characters \u{1F4AF} instead of the syntax \U0001F4AF (recognised in Python world).

The rule CodePoint (named after ECMAScript grammar) recognises the following sequences:

• \u{0+} e.g. \u{00000000000000} resulting in the Unicode character U+0000
• \u{0*HexDigit{1,4}} resulting in the Unicode character in BMP
• \u{0*(10|HexDigit)HexDigit{4}} resulting in the Unicode character in astral plane

This PR or #586 have a similar goal but a different proposed syntax.

They are both limited to constant strings and do both a bad job in characters classes like [a-\u{1F4AF}]. Characters classes should be treated by further PRs, see my comment for other points.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/peg-js/pegjs/ewy36hurn
✅ Preview: https://pegjs-git-fork-seb35-651.peg-js.now.sh

[hildjj]

Can you port this over to merge cleanly on peggyjs/peggy please? I'd like this to be in the first peggy release.

[hildjj]

Actually, not before peggy 1.0. See: peggyjs/peggy#15