cell sublanguage: a[i] support in -n (nameref) and ${!ref}

[andychu]

• usage from @abathur https://oilshell.zulipchat.com/#narrow/stream/121540-oil-discuss/topic/nameref.20implemented
• I think bash-completion used it in the ${!ref} context, although I may have patched around it

[andychu]

Details: the ${!ref} code in osh/word_eval.py should be changed to use a regex-based lexer to recognize a[K] where K is a constant.

Then it can be shared with the -n code in core/state.py

We probably don't want to support a[i+1+2*3], etc. because that's too much dynamic parsing

[andychu]

May relate to IR in #604 , e.g. sh_lhs_expr

[andychu]

This is a dupe of #964 and #881

var ref is done, but nameref is a bit annoying