CSS selection rules should include hyphens and underscores

[slodge]

Fixes #355

Extend the current regexes to allow selections of examples like:

• <my-hyphened-tag />
• <my_underscored_tag />
• <div class="my-hyphened-class" />
• <div class="my_underscored_class" />
• <div id="my-hyphened-id" />
• <div id="my_underscored_id" />
• <my-hyphened-tag id="my_underscored_id" class="my_underscored_class my-hyphened-class" />

These are valid identifiers based on https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier

Todo:

• [x] Change the selector.R code and test locally
• [x] Update tests
• [x] Update NEWS.md
• [x] Link to an issue
• [x] Sign CLA (waiting on Legal at work - should be quick)

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
2 out of 3 committers have signed the CLA.

:white_check_mark: slodge
:white_check_mark: schloerke
:x: Stuart Lodge

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[slodge]

+ 6323da93a22f171f2810657ea07b29aaaa44de80 `devtools::document()` (GitHub Actions)
Error: Some auto-generated commits were found. GHA is not allowed to push commits back to a forked repo.
Error: Please perform and commit these actions manually. It may help to merge in the latest changes from the base branch.
Error: Process completed with exit code 1.

Saw this error... have pushed some changes - but they're nothing to do with this commit...

Personally I'm very happy with GitHub Actions running document instead of developers - sounds like a perfect place for automation to reduce work rather than adding to it :)

(insert post-xmas back-at-work fat and grumpy Stuart emoji here)

[schloerke]

GHA routine issue is unrelated to this PR