Redesign and rewrite some parts of `porcupine.settings`

[Akuli]

I'm no longer happy with some of the things it does. There are many small problems, so I decided to just make one issue about them.

In #1193, I discovered that the JSON serializing code is leaking memory. I should replace the class-in-function hack it's using now with something much simpler.

To merge tab settings from multiple places, such as .editorconfig and filetypes.toml, it would be nice to provide a priority when settings are set. Then the .set() with highest priority would be used. For example, the editorconfig plugin would use a higher priority than the filetypes plugin, so that it overrides whatever filetypes sets. The loading order is no longer enough, because I want to reload filetypes.toml configs without reloading editorconfigs, but things set in editorconfig should still take precedence.

In the past, benjamin-kirkbride has been confused by settings.get("setting_name", list[str]). It looks somewhat like dict.get, but behaves differently.

If a setting can be None, you can still .get() it with a non-None type whenever it isn't set to None. You then get a runtime error when the value is None. See #1440 for an example.