Highlight parameter expansion history modifiers

[danielshahaf]

$foo:hey should highlight $foo:h as a parameter expansion.

• both within and without quotes
• modifiers can be chained, $foo:h:h:t

[danielshahaf]

Modifiers require braces when KSH_ARRAYS is set.

[danielshahaf]

According to zshexpn, modifiers are:

• :[aAcehlpPqQ&tuxfw]
• :s/l/r[/] , :gs/x/y/, :s/x/y/:G where the delimiter can vary
• :F:expr: where the delimiter can vary
• :W:sep: likewise

[danielshahaf]

@phy1729 You mentioned having some pending changes to this area of the code, please unassign yourself once they've been merged to master (to avoid needless conflicts). Thanks!

[danielshahaf]

I've implemented something that covers the modifiers that take no argument: https://github.com/zsh-users/zsh-syntax-highlighting/compare/master...danielshahaf:i485-history-modifiers-v1

... but if I wrote a regression test, it would conflict with #492, so instead I just described the regression test I would create (pending manual verification, of course) and I'll put this on hold pending #492's merge.

[phy1729]

I think that bit and the parameter handling in _highlight_arguments should be pulled out into a new function. Not sure what to do about the style. Perhaps parameter with a fallback to dollar-double-quoted-argument?

[danielshahaf]

dollar-un-quoted-argument (maybe without that ungrammatical hyphen) would be consistent. variable or parameter-expansion would work.

[danielshahaf]

Triage: https://github.com/zsh-users/zsh-syntax-highlighting/pull/697#issuecomment-632442416