feat(parser): accept list literals in field-function arguments

What does this MR do and why?

Third of three MRs for list-valued field-function parameters. Stacked on !520 (merged) (typed values) and !521 (merged) (list definitions). This is the language change.

Syntax: name(key=[v1, v2, ...]) and name([v1, v2]), items being the scalar forms field functions already accept (quoted strings, identifiers, numbers); [] is the empty list. Brackets rather than the parenthesised list filters use, because inside a call (...) would read as a nested function. [ has no other use in the grammar. Numbers may be negative (foo(-1), tiers=[-10, 0, 10]), matching what the integer validators accept.

• Malformed lists (unterminated, trailing comma, empty item, nested list, missing separator) fail to parse; FieldFunction keys with list items re-parse to the same value.
• Several positional values for a single list parameter (userTier(4, 25, 100)) get an error pointing at the bracketed spelling instead of the generic arity message.
• The List value kind in schema.json documents both spellings: parenthesised in a filter, bracketed in a field-function parameter. That is the only change to the document.
• to_sentence_case, which labels every field-function argument and list item, handles a multi-byte first character and inputs without word characters instead of panicking.

The first users are userTier(thresholds=[...]) in !516 (merged), stacked on this MR, and totalCount(status=[...]) for #215 (closed); end-to-end examples live there.

Follow-ups outside this repo

• Grammar docs for field functions live in gitlab-org/gitlab (doc/user/glql/) and need the list syntax once a release ships it.
• The monolith's labelWithParameter (app/assets/javascripts/glql/utils/chart_data.js) stringifies an array parameter as 4,25,100 in the label suffix (aliasing hides it); flattening the parameter values before stringifying is a cosmetic follow-up alongside the bump.

How to review

src/parser/fields.rs (list_arg_value, scalar_arg_value, the tests), src/utils/common.rs.

Closes #213 (closed)