ci: vitest workspaces (#998)

## 🧰 Changes

> [!IMPORTANT]
> **this continues to be a vitest supremacy account 😤**

In this repo, we have tests that use `process.chdir()`, so we maintain a
separate Vitest config for running those via a
[`forks`](https://vitest.dev/config/#forks) pool vs. the standard
[`threads`](https://vitest.dev/config/#threads) pool — see
#870 for more background on this.

We've used two separate npm scripts (i.e., `npm run test:multi && npm
run test:single`) to run our tests sequentially. I discovered this
morning that [Vitest's workspaces
feature](https://vitest.dev/guide/workspace) isn't just for monorepos —
it's actually the recommended way to maintain multiple test
configurations no matter the context.

In this PR, I consolidated our two test configs into a single workspace
config. We get a few nice benefits by leveraging workspaces:
- No longer needing to manually pass in the correct config file when
running certain test files (e.g., `npx vitest --config
vitest.single-threaded.config.ts [file]`). In other words, `npx vitest
[file]` ✨ just works ✨, which should make it a lot easier for new
contributors to run tests
- Much cleaner test output — we now can see coverage across all files,
rather than two separate coverage reports[^1]
- Easier to share options between the two configs
- A single config file to rule them all
- Tests seem to be faster (but hard to say)

## 🧬 QA & Testing

Do all the tests run? And do they still run and pass properly? Here's
the latest test run in `next` for comparison:
https://github.com/readmeio/rdme/actions/runs/8910638856

[^1]: Small downside to this — [workspaces don't allow you to configure
coverage options](https://vitest.dev/guide/workspace#coverage). Not a
huge deal since our coverage is great in this repo and we can stick to
the defaults and it still addresses our needs.

package.json

@@ -119,9 +119,7 @@
     "prettier": "prettier --check .",
     "schemas:check": "./bin/json-schema-store.js",
     "schemas:write": "./bin/json-schema-store.js --update",
-    "test": "npm run test:multi && npm run test:single",
-    "test:multi": "vitest run --coverage",
-    "test:single": "vitest run --coverage --config vitest.single-threaded.config.ts"
+    "test": "vitest run --coverage"
   },
   "commitlint": {
     "extends": [

vitest.workspace.ts

@@ -0,0 +1,48 @@
+// eslint-disable-next-line import/no-extraneous-dependencies
+import { defineWorkspace } from 'vitest/config';
+
+const sharedExclude = [
+  '**/dist/**',
+  '**/node_modules/**',
+  '**/__fixtures__/**',
+  '**/helpers/**',
+  '**/__snapshots__/**',
+];
+
+const sharedOpts = {
+  globalSetup: ['./__tests__/setup.js'],
+  setupFiles: ['./__tests__/helpers/vitest.matchers.ts'],
+};
+
+export default defineWorkspace([
+  {
+    /**
+     * Our default test suite
+     */
+    test: {
+      exclude: [...sharedExclude, '**/single-threaded/**'],
+      name: 'default',
+      ...sharedOpts,
+    },
+  },
+  {
+    /**
+     * Our single-threaded test suite.
+     * Only tests that use `process.chdir()` should be run using this config.
+     */
+    test: {
+      exclude: sharedExclude,
+      include: ['**/single-threaded/**'],
+      name: 'single-threaded',
+      /**
+       * We can't run tests with `threads` on because we use `process.chdir()` in some tests and
+       * that isn't available in worker threads, and it's way too much work to mock out an entire
+       * filesystem and `fs` calls for the tests that use it.
+       *
+       * @see {@link https://github.com/vitest-dev/vitest/issues/566}
+       */
+      pool: 'forks',
+      ...sharedOpts,
+    },
+  },
+]);