Add JS package with pattern parsers & serializers

.github/workflows/js.yaml

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+name: JS
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - run: npm ci
+        working-directory: ./js
+      - run: npm test -- --coverage
+        working-directory: ./js
+      - run: npm run build
+        working-directory: ./js
+
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: npm ci
+        working-directory: ./js
+      - run: npm run lint
+        working-directory: ./js
+      - run: npx prettier --check .
+        working-directory: ./js

js/.gitignore

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+/coverage/
+/dist/
+/node_modules/

js/README.md

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+# @mozilla/l10n
+
+This is a library of JavaScript tools and utilities for working with localization files,
+primarily built for internal use at Mozilla.
+
+It's being bootstrapped to fulfil the needs of the [Pontoon](https://pontoon.mozilla.org/) frontend,
+and to match the behaviour of the Python [moz.l10n](https://pypi.org/project/moz.l10n/) library.
+
+The data structures used by the library are defined by a
+[JSON Schema](https://github.com/mozilla/moz-l10n/blob/main/moz/l10n/message/schema.json).
+
+The library currently supports the following message pattern formats:
+
+- `android`: Android string resources
+- `fluent`: Fluent (without internal selectors)
+- `mf2`: MessageFormat 2.0
+- `plain`: Patterns without placeholders (used in multiple resource formats)
+- `webext`: WebExtensions (messages.json)
+- `xliff`: XLIFF 1.2, including XCode customizations
+
+The tooling for `android` and `xliff` depends on `DOMParser`, `XMLSerializer`, and related classes
+which are available in browser environments.
+The parser for `fluent` depends on the `@fluent/syntax` package,
+and the parser for `mf2` depends on the `messageformat` package.
+
+## API
+
+### parsePattern()
+
+```js
+import { ParseError, parsePattern } from '@mozilla/l10n'
+```
+
+```js
+function parsePattern(
+  format: 'android' | 'fluent' | 'mf2' | 'plain' | 'webext' | 'xliff',
+  src: string,
+  baseMsg?: Message,
+  onError?: (error: ParseError) => void
+): Pattern
+
+class ParseError extends Error {
+  range?: [number, number]  // Set for fluent, mf2, and webext errors
+}
+```
+
+Parse the string representation of a single flat message pattern into a data structure
+([JSON Schema](https://github.com/mozilla/moz-l10n/blob/main/moz/l10n/message/schema.json)).
+
+The `baseMsg` is required by `webext` for named placeholders.
+
+If `onError` is undefined, errors are thrown.
+
+### serializePattern()
+
+```js
+import { SerializeError, serializePattern } from '@mozilla/l10n'
+```
+
+```js
+function serializePattern(
+  format: 'android' | 'fluent' | 'mf2' | 'plain' | 'webext' | 'xliff',
+  pattern: Pattern,
+  onError?: (error: SerializeError) => void
+): string
+
+class SerializeError extends Error {
+  pos?: number  // Set for fluent, plain, and webext errors
+}
+```
+
+Serialize the data representation of a single flat message pattern into a string
+
+If `onError` is undefined, errors are thrown.
+
+### Types
+
+```ts
+import type {
+  Message,
+  PatternMessage,
+  SelectMessage,
+  Pattern,
+  Expression,
+  Markup
+} from '@mozilla/l10n'
+```
+
+```ts
+type Message = Pattern | PatternMessage | SelectMessage
+type Pattern = (string | Expression | Markup)[]
+```

js/eslint.config.mjs

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+// @ts-check
+
+import eslint from '@eslint/js'
+import prettier from 'eslint-config-prettier'
+import tseslint from 'typescript-eslint'
+
+export default tseslint.config(
+  eslint.configs.recommended,
+  tseslint.configs.recommended,
+  prettier
+)