Another large rewrite

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

For some reason whenever I work on language-yaml I end up rewriting the whole thing each time. Oh well. This PR should probably have been split up into like 10 different ones, but many of the changes depend on the others.

• Pulled standalone - stuff (aka tags without a colon) into its own match
  • This greatly simplifies the main tag match, meaning that we no longer have three nested non-capturing groups
• Ensure that spaces follow the ending colon (fixes the second part of Incorrect syntax highlights #53)
• Use a consistent blacklist across tag matches (fixes the first part of Incorrect syntax highlights #53)
  • Note: somehow I messed up the issue number twice in the commit. The one listed here is correct.
• Fix constant.other.date.yaml being misapplied to the entire match, rather than just the date
• Yank date and numeric matching into a repository so that they can be included when necessary, rather than making custom matches for them that would only work under certain scenarios
• Integrate !!omap matching into the existing tag matches
• Make variable matching stricter
• Moved many rules (comments, strings, variables) into the repository to reduce code duplication and maintenance
• Properly match non-specific tag indicators (key: ! value)
• Don't match leading whitespace as an unquoted string
• Properly match merge-key tags (<<: *variable-to-merge)
• Remove old rules that were never used

Alternate Designs

None. This is an overall rewrite to the grammar to hopefully make it easier to maintain in the future. Code duplication was getting out of hand, and many of the more obscure rules weren't working properly.

Benefits

Better overall code tokenization and accuracy.

Possible Drawbacks

Possible regressions, as is possible with any rewrite. Spec coverage has been improving each rewrite though, so the chance of severe regressions happening shouldn't be that large anymore.

Applicable Issues

Fixes #53

[winstliu]

@MaximSokolov: do you think this scope name is ok, or would switching it to a keyword like keyword.other.tag.non-specific.yaml be better?

[MaximSokolov]

I don't know. Seems to me it's more than just a punctuation mark. According to documentation (3.3.2. Resolved Tags), there is also a ? tag, does it have a scope?

[winstliu]

Continuing the conversation here so that it's more visible: no, it currently doesn't. The ! tag was already there, so I kept it, but didn't add any of the other ones. That said, they look similar, so I could just group them together: [?!](?=\\s)

[MaximSokolov]

I am concerned about using punctuation.definition.keyword.yaml scope name for !!, because syntax--keyword selector can override styles for syntax--punctuation. It should be changed either to punctuation.definition.smth-else.yaml or keyword.other.smth.yaml.

As for ! tag, I'll leave final decision to you, as I am not quite familiar with YAML syntax

[winstliu]

punctuation.definition.tag.omap?

[MaximSokolov]

punctuation.definition.tag.omap

👍

[winstliu]

I think I'm going to avoid adding more scopes in this PR. Those can be added separately.