Add unit tests with vitest (#15)

Vitest as a devDependency. New tests/ directory covers the four
audit-identified pure-function targets:

- duration.test.ts — formatDuration thresholds (ms/s/m/h),
  negatives, fractional clamp
- pluralize.test.ts — 1 vs n, custom plurals, negatives
- snapshotParser.test.ts — flat list, no-label rows, escaped-quote
  labels, depth from leading whitespace (spaces + tabs), ignored
  lines, dedupe, empty input, CRLF
- scriptParser.test.ts — parseScript (blank/comment-only, context
  and env directives, action line numbers, comment stripping
  outside strings, escaped quotes inside quoted tokens, CRLF) +
  interpolate (basic, inside quotes, default fallback, defined
  beats default, missing → empty, non-vars untouched) + dequote
  (strip + unescape, unquoted passthrough, unbalanced quote)

35 specs total, ~130ms runtime.

To make parseSnapshotRefs importable without VS Code at runtime,
extracted it to src/runners/snapshotParser.ts (no vscode imports).
src/services/snapshotIndex.ts re-exports it for existing callers.

CI runs `npm test` between format:check and compile.
package.json scripts: test (one-shot), test:watch.
.vscodeignore excludes tests, .github, lint/format/test configs
from the published vsix.

Author: azizbecha

.github/workflows/ci.yml

@@ -22,6 +22,8 @@ jobs:
 
       - run: npm run format:check
 
+      - run: npm test
+
       - run: npm run compile
 
       - run: npm run package

.vscodeignore

@@ -1,6 +1,12 @@
 .vscode/**
 .vscode-test/**
 src/**
+tests/**
+.github/**
+.oxlintrc.json
+.oxfmtrc.json
+vitest.config.ts
+examples/**
 .gitignore
 .gitattributes
 .git/**