improve handling of Sexpr for sweave

[kevinushey]

Intent

Addresses #14382.

Approach

Update the Sweave highlight rules, to use the new (and more robust) context object for storing tokenizer state.

Automated Tests

N/A

QA Notes

Test via notes in #14382.

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!

[nickreich]

Thanks for fixing this!