Fix 'oneof' in syntax-directed operations

[szuend]

In the source map spec we have the use-case of declaring an SDO over a lexical production (for VLQ decoding).

This PR adds the missing implementation.

[michaelficarra]

This should read

Suggested change

	Foo :: one of \`a\`
	Foo :: \`a\`

We should prohibit "one of" with only one alternative.

[szuend]

I changed the test to Foo :: one of `a` `b` instead. Allowing Foo :: `a` should probably be done in a separate PR since it requires changing rhsMatches to compare different kinds of productions. Also disallowing Foo :: one of `a` should probably be done in the linter in a separate PR.

[jmdyck]

Note that the current spec does have examples of an SDO being defined on a "one of" production, but they're all in the 'inline' form, rather than the emu-grammar+emu-alg form:

- The TRV of <emu-grammar>DecimalDigit :: one of `0` `1` `2` `3` `4` `5` `6` `7` `8` `9`</emu-grammar> is ...
- The TRV of <emu-grammar>HexDigit :: one of `0` `1` `2` `3` `4` `5` `6` `7` `8` `9` `a` `b` `c` `d` `e` `f` `A` `B` `C` `D` `E` `F`</emu-grammar> is ...
- The TRV of <emu-grammar>SingleEscapeCharacter :: one of `'` `"` `\` `b` `f` `n` `r` `t` `v`</emu-grammar> is ...

[szuend]

Anything missing to get this landed?

[bakkot]

Published in 21.1.0, sorry for the delay.