Bring in data-directory, let's go async file reads (#16782)

* Bring in data-directory, let's go async file reads

* Lint fixes

* Update glossary.js

tests/unit/data-directory/filename-to-key.js

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+const filenameToKey = require('../../../lib/filename-to-key')
+
+describe('filename-to-key', () => {
+  test('converts filenames to object keys', () => {
+    expect(filenameToKey('foo/bar/baz.txt')).toBe('foo.bar.baz')
+  })
+
+  test('ignores leading slash on filenames', () => {
+    expect(filenameToKey('/foo/bar/baz.txt')).toBe('foo.bar.baz')
+  })
+
+  test('supports MS Windows paths', () => {
+    expect(filenameToKey('path\\to\\file.txt')).toBe('path.to.file')
+  })
+})

tests/unit/data-directory/fixtures/README.md

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+I am a README. I am ignored by default.

tests/unit/data-directory/fixtures/bar.yaml

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+another_markup_language: 'yes'

tests/unit/data-directory/fixtures/foo.json

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+{"meaningOfLife": 42}

tests/unit/data-directory/fixtures/nested/baz.md

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+I am markdown!

tests/unit/data-directory/index.js

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+const path = require('path')
+const dataDirectory = require('../../../lib/data-directory')
+const fixturesDir = path.join(__dirname, 'fixtures')
+
+describe('data-directory', () => {
+  test('works', async () => {
+    const data = await dataDirectory(fixturesDir)
+    const expected = {
+      bar: { another_markup_language: 'yes' },
+      foo: { meaningOfLife: 42 },
+      nested: { baz: 'I am markdown!' }
+    }
+    expect(data).toEqual(expected)
+  })
+
+  test('option: preprocess function', async () => {
+    const preprocess = function (content) {
+      return content.replace('markdown', 'MARKDOWN')
+    }
+    const data = await dataDirectory(fixturesDir, { preprocess })
+    expect(data.nested.baz).toBe('I am MARKDOWN!')
+  })
+
+  test('option: extensions array', async () => {
+    const extensions = ['.yaml', 'markdown']
+    const data = await dataDirectory(fixturesDir, { extensions })
+    expect('bar' in data).toBe(true)
+    expect('foo' in data).toBe(false) // JSON file should be ignored
+  })
+
+  test('option: ignorePatterns', async () => {
+    const ignorePatterns = []
+
+    // README is ignored by default
+    expect('README' in await dataDirectory(fixturesDir)).toBe(false)
+
+    // README can be included by setting empty ignorePatterns array
+    expect('README' in await dataDirectory(fixturesDir, { ignorePatterns })).toBe(true)
+  })
+})