[Digit separators] Syntax highlighters implementation

[srawlins]

• https://github.com/dart-lang/dart-syntax-highlight/ (@DanTup, Add support for digit separators dart-syntax-highlight#73)
• https://youtrack.jetbrains.com/issue/IDEA-356065/Support-Dart-digit-separators-in-number-literals
• google3
• https://github.com/highlightjs/highlight.js (Update Dart for digit-separators feature highlightjs/highlight.js#4081)
• https://github.com/codemirror/CodeMirror/ CodeMirror 5 is deprecated and CodeMirror 6 has no support for Dart. But DartPad runs on CodeMirror 5, so we still need support here. Dart: support digit separators codemirror/codemirror5#7105

[srawlins]

I think we should skip CodeMirror (and can skip it for language features going forward).

AFAICT there is only existing Dart support for CodeMirror 5. For CodeMirror 6 there is a new way to specify a language mode, a new way to specify the parser, and there is no support for Dart as a language in CodeMirror 6. It would be great to add that support, but as for the Digit separators feature, I think it is not important to bump the support for CodeMirror 5, and a separate effort to wire up CodeMirror 6.

[bwilkerson]

@anderdobo @mossmana

[srawlins]

I added back CodeMirror 5 since DartPad actually still runs on CodeMirror 5.

[srawlins]

Calling this done. google3 can be tracked separately.