use separate highlight rule set for chunks / Sexpr

[kevinushey]

Intent

Addresses #14667.
Addresses #14646.

Approach

Rather than use the same highlight rules (and so, same highlighter state) for both R code chunks and \Sexpr{} code blocks, use separate highlighter sets. This is necessary as we need to track the { and} count within \Sexpr{} calls, but not within regular R code chunks.

Automated Tests

Included in PR.

QA Notes

Test via notes in associated issues.

Documentation

N/A

Checklist

• If this PR adds a new feature, or fixes a bug in a previously released version, it includes an entry in NEWS.md
• If this PR adds or changes UI, the updated UI meets accessibility standards
• A reviewer is assigned to this PR (if unsure who to assign, check Area Owners list)
• This PR passes all local unit tests

[gtritchie]

LGTM!