Adds a lexer for Roc, requested in #3364.

It covers the current main syntax: \ interpolation with nested braces, \\\\ block strings, \\u(....) unicode escapes, and/or word operators, the ?/?? postfix operators, .. rest/update patterns and ..= ranges, nominal (:=) and opaque (::) types with .{ } extension blocks, .U8-style number suffixes, and \$-prefixed var names. A lowercase name starting a line is highlighted as a definition.

Tests are five snippets under tests/snippets/roc/ plus Roc's own all-syntax test (from roc-lang/roc's test/echo/all_syntax_test.roc) as an examplefile, so the golden output tracks their syntax reference directly.

On the community bar in the contributing guide: Roc is pre-0.1, but it has a 501(c)(3) foundation, corporate sponsorship, an Exercism track and active editor tooling, and Zulip's markdown renderer is waiting on this for roc code fences (zulip/zulip#40325) - I think it clears the bar.