Save extension settings to `browser.storage`

[ACTCD]

Ref: #264 (comment)

Current extension settings and index information, etc. are stored in:
~/Library/Containers/com.userscripts.macos.Userscripts-Extension/Data/Documents/manifest.json

It writes to the filesystem through a native application layer (Swift) proxy.

Saving them rescheduled to browser.storage.local has many benefits:

• Directly use the browser extension storage API to reduce extension complexity.
• The settings can be directly read in the content script, reducing a layer of message relay.
• Prepare scripts, indexes and other information in advance to improve the injection speed.
• Use its asynchrony to improve interface response and access flexibility.
• Store script update information and persist update check data for front-end use.

[quoid]

This would be a major improvement. The use of manifest,json comes from previous versions where we didn't have access to the WebExtension API. This would further reduce the number of disk read/writes which is an overall good thing.