support for labeling steps

[bakkot]

This introduces the ability to label steps, towards a solution to tc39/ecmarkup#192.

Corresponding upstream ecmarkup PR at tc39/ecmarkup#217. Corresponding upstream ecma262 PRs at tc39/ecma262#2052.

The ecmarkup PR describes how this is intended to be used, and the ecma262 PR shows what that looks like for ecma262 master.

Fixes #56.

[michaelficarra]

Is there some escape hatch that would allow us to write a step whose text begins with what looks like a label? It's not a big deal, but it would be nice.

[michaelficarra]

Should we try to look for a label following an unordered list item so we can fail with a useful message instead of passing it through as text?

[michaelficarra]

Actually, now that I think about it, it could be useful to label an unordered list item. For example, linking to the step via a URL fragment.

[bakkot]

Actually, now that I think about it, it could be useful to label an unordered list item. For example, linking to the step via a URL fragment.

Sure, makes sense. Updated; want to take another look?

[jmdyck]

Is there some escape hatch that would allow us to write a step whose text begins with what looks like a label? It's not a big deal, but it would be nice.

In the ecma262 spec, I'm pretty sure there's no step-syntax currently in use that would allow a step to start with a left square bracket, so it's unclear what you'd need the escape hatch for.

(In practice, the vast majority of steps start with an uppercase letter. The only exceptions I can find are:

• 3 occurrences of <emu-not-ref>Record</emu-not-ref> that ...
• 2 steps in the definition of the ? shorthand that (unrealistically) start with ?
• Step 14.a in RegExpBuiltinExec, which begins _e_ is an index into the _Input_ character list, which should probably extracted into a NOTE step.

)