Fix extension grammar: multi-char operators (=>, ->, …) highlight as one token

[assapir]

Problem

In the Quilon VS Code grammar, multi-character operators risked being highlighted as two tokens (e.g. => could color the = in one scope and the > in another), the same for ->, :=, |>, <-, ==, !=, <=, >=, &&, ||, ::. TextMate applies, at each position, the first pattern in a rule list that matches there — ties at the same start position are decided by list order, not match length — so a single-character operator rule ordered ahead of a multi-character one would split the operator.

Fix

In syntaxes/quilon.tmLanguage.json, make every multi-character operator win as a single token:

• Order all multi-char operator rules before the single-char rules, and before any single-char operator that is a prefix of them (|>/|| before |, <= before <, =>/== before =, etc.). Each multi-char operator keeps exactly one scope.
• Split the old &&|\|\||! rule so the two-char &&/|| sit in the multi-char tier while the single-char ! stays in the single-char tier (keeps != correct).
• << / >> remain single import/export tokens (unchanged module-line rules).
• No regression to $ (unit), ~ comments, </> (comparison + block delimiters), strings, or numbers.

Tests (mandatory)

Added src/grammar.test.ts, backed by a tiny dependency-free TextMate match-engine (src/grammar.ts) that reproduces the ordered first-match-wins rule (no native vscode-textmate/vscode-oniguruma dependency, so it runs under the existing node --test gate). It asserts:

• each multi-char operator (=>, ->, :=, |>, <-, ::, ==, !=, <=, >=, &&, ||) tokenizes to a single, consistent scope — both spaced (a => b) and tight (a=>b);
• <</>> stay single tokens; adjacent operators (a==b!=c<=d>=e) each stay one token; a representative lambda + arrow-type line highlights each operator once;
• regression guards for </>, =, $, comments, strings, and numbers.

Verified the test is meaningful: with the buggy (single-char-first) ordering, 19 of these assertions fail; with the fix they all pass.

Updated the extension README to document the single-token behavior and the new grammar tests.

Verification

pnpm install --frozen-lockfile, then pnpm run lint, pnpm run fmt:check, pnpm test (51 passing), and pnpm run package (.vsix builds) — all green. Scope is limited to the grammar JSON, the grammar tests, and the README; no package.json/lockfile/.npmrc/workflow/Rust changes.

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