Handle special characters in tags

[JWCook]

Closes #39; closes #56; closes #58

Changes

• Normalize output filenames (for nicer URLs without urlencoded characters)
• Allow spaces in tag labels
• Allow emoji in rST tag labels
  • After making tests, I found that rST header underlines were too short if any emoji (or other "wide" unicode characters) are in the tag label.
  • For reference: this can be handled with unicodedata.east_asian_width() (width category per character) or sphinx.util.rst.textwidth() (total width per string).
• Exclude source files that match config.exclude_patterns
• Sort tag list for MyST tag pages (currently only done for rST)

Example

A tag named {{🧪test tag; please ignore}} will result in:

• test-tag-please-ignore.md / .rst
• test-tag-please-ignore.html
• Tag name is unchanged everywhere else (as displayed on tag index, tag pages, and pages with tags)

[JWCook]

@melissawm These changes are ready for review now.

[JWCook]

I moved all tests for this PR to #59 to try to make this a bit easier to review.

[speth]

@melissawm - Is there a chance for this to be reviewed / merged soon? I'm interested in using sphinx-tags and wanted to be able to use tags with spaces. I fixed that and another small problem in my fork (speth/sphinx-tags) but didn't want to clobber this work that includes several other improvements.

[melissawm]

I am sooooo sorry it's taken me this long - real job caught up with me 😅 I will review now.