Refactor getMacroMatches & fix a couple of bugs

[njvrzm]

What this PR does / why we need it:

This refactors and updates getMacroMatches and parseArgs to remove duplicated logic and address two bugs.

Refactoring: previously getMacroMatches and parseArgs did quite a bit of overlapping work: getMacroMatches would parse the input string to find a bracketed substring string representing macro arguments, then parseArgs would take that string and parse it again to extract the arguments. The logic was also implemented differently in each. I've moved all the work of parsing arguments into parseArgs.

Bugs addressed:

• Macros can consume too much of a query containing parentheses #859, an extension of Macro matching is too lax sqlds#82, where a bracketed substring was treated as an argument list when it shouldn't be
• getMacroMatches panics if given a macro arg string missing its close bracket #861, where a badly-formed macro argument string would cause getMacroMatches to panic

Which issue(s) this PR fixes:

Fixes #859, #861.

Special notes for your reviewer:

Once the base branch (858-changes_from_sqlds) is merged I'll retarget this to main.

[gabor]

LGTM 👍