Meta: bump ecmarkup to 15.0.0 and apply new formatting/lint rules

[bakkot]

Depends on #2896, and is based on that branch.

I recommend reviewing commits individually, and commits should land individually.

The "Markup: replace HTML entities" commit was generated by running npm run format; no manual changes were applied. Sidebar: we sure do have a lot of fancy quote marks.

The "Markup: mark certain non-obvious variable declarations" commit is necessary to get the linter's new "variables must be defined" lint to pass; the ten lines it touches make use of variables in non-obvious ways that I didn't want to teach the linter about.

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This has landed and will inevitably cause merge conflicts with almost everything, but fear not, you don't need to deal with it manually. To rebase a branch onto this, you can use the following commands:

git rebase -i 0090daf^ # 0090daf is the commit where the formatter is applied
git rebase -X theirs 0090daf --exec "npm i && npm run format && git commit --amend -a --no-edit"
git rebase -i main

This splits up the rebase into 1) everything before the formatter was applied, 2) the formatter, which will be done for you, 3) everything after the formatter. any conflicts in parts 1 and 3 you will need to handle manually, just like you would in the absence of the formatter (or feel free to ask the editors for help).

[michaelficarra]

FYI see #2902

[michaelficarra]

LGTM but I'd like to see #2902 and #2911 resolved first.

[bakkot]

This now incorporates changes necessary to make #2902 pass. It also adds a new marked non-obvious declaration for an alias introduced in #2781 (in a fixup commit, which should get squashed into the "mark certain non-obvious variable declarations" commit when landing).

It's ready to go as soon as #2896 lands, and will then un-block #2902.