Add utilities for operating with version metadata

[pan93412]

This commit introduces two new tools:

• lint-version-metadata.ts: Automatically check if both the dlfx_var.shtml and the dltb2.shtml are defined correctly.
• get-stable-version.ts: Get the latest stable version of Firefox and Thunderbird from API.

Both the tools can (in my opinion) largely improve the DX to update the version of Firefox and Thunderbird, while not being too automatic to preserve the current version-updating workflow.

The both tools need only Deno runtime to run, and have limited the permission they can retrieve. The detailed usage information has embedded in the code, and you can refer to them when needed.

To Be Discussed

• [x] Should we document this tool in README.md? I'm not sure if README.md is suitable to place this.

[irvin]

看起來目前 script 本身的說明 ok

或許可以在 https://github.com/moztw/www.moztw.org/blob/master/inc/README.txt 補上「更新版號的 SOP」的段落，提示有這個 script 可以使用