Dart: support digit separators

[srawlins]

Dart is adding support for digit separators, the underscore, like 1_000_000 or 0x33_33_33. The Dart playground, DartPad, still runs on codemirror5, so this would be a welcome fix.

[marijnh]

That regexp seems to allow underscores anywhere, whereas the linked proposal only allows them between digits. I think this'll make _ tokenize as a number, which doesn't seem right.

[srawlins]

As I understand it, the number pattern is only checked when we see numberStart, line 65 in clike.js. With manual testing, I see that _ is styled as a "variable."

As for other spots where _ is not allowed (e.g. in 1_._0, neither underscore is allowed), I think it is fine to allow them for the purposes of in-progress typing, and error-recovery. For example, in the Dart parser, we parse 1_._0 as a number literal, and also report an error. This reduces errors-carried-forward. And for a trailing _ like 100_, it is fine to allow it for code-in-progress.

If you think we should be more strict here and only parse error-free numbers as numbers, I can amend the PR.

[marijnh]

Oh, indeed, there's a separate regexp for the start of the number. Then this is close enough, indeed.