Render KaTeX by default

[gnprice]

Fixes #46. (By enabling the TeX support whose substance was added over a number of preceding PRs.)

Here's a version of the commit I've included in each release recently, atop (previous versions of) #1559, to enable our KaTeX support. Now that #1559 and its preceding PRs (#1698, and others before that) are merged to main, this is ready to go to main too.

This was a minimal version for the release, just flipping the flag's default to true. But in general we should remove experimental flags from the codebase as soon as they're no longer relevant, to keep complexity from building up. (See the doc on [GlobalSettingType.experimentalFeatureFlag].) @rajveermalviya would you write a follow-up commit that removes this flag entirely, and cleans up the logic elsewhere which that makes possible to clean up?

5287d39 content: Render KaTeX by default, the way we have in recent releases

This is cherry-picked from commits we included in each of the last
several releases, starting with 8f37237 in v0.0.31.

[gnprice]

@rajveermalviya would you write a follow-up commit that removes this flag entirely, and cleans up the logic elsewhere which that makes possible to clean up?

I've filed #1728 to track this follow-up task.

Merging this change now, in the interest of further syncing up the main branch with what we've had in recent releases.