File System Access per sycxyc

This extension brings the File System Access API to Firefox that helps web apps such as https://vscode.dev read and write local files and folders.

Main features:
* Implemented showOpenFilePicker, showDirectoryPicker, showSaveFilePicker functions and related interfaces.
* Set to enable specific File System Access features on matching web pages.
* Provides File System Access API for other compatible WebExtensions.

Notes:
* The local file operations required by this extension cannot be performed in the browser, and a helper app needs to be installed to assist in the related work.
* The optional Code Editor feature is provided by the Code Editor extension.

Limitations:
* By default, FileSystemHandle will lose its instance methods after cloning (e.g. using IndexedDB or postMessage), and requires additional configuration of the FS_CONFIG.CLONE_ENABLED. Web developers can use __FILE_SYSTEM_TOOLS__.parseHandle(handle) to restore the instance methods.
* Limited Worker context support and requires additional configuration of the FS_CONFIG.WORKER_ENABLED.
* Read file size is limited by the FS_CONFIG.FILE_SIZE_LIMIT. Web developers can read large file streams and slices with handle.getFile({ _allowNonNative: true }), and write large file in-place with handle.createWritable({ _inPlace: true, keepExistingData: true }).
* DataTransferItem.prototype.getAsFileSystemHandle is not implemented.

This extension is open source and you can file bug reports or feature requests on the GitHub issue.

References:
* https://wicg.github.io/file-system-access/