Refactor shell theming module

[lavafroth]

Description

This PR refactors a few operations to make the code handle edge cases efficiently with marginal performance improvements.

Type of change

• Bugfix (Change which fixes an issue)
• New feature (Change which adds new functionality)
• Enhancement (Change which slightly improves existing code)
• Breaking change (This change will introduce incompatibility with existing functionality)

Changelog

• Stores the comma separated string representation of valid versions for later use in raising exceptions.
• Uses tuple for async_data in ShellTheme since its members are immutable.
• Uses tuple decomposition to assign theme variant and preset from async_data later.
• Replaces Gio.File.make_directory_with_parents with Python's builtin os.makedirs with exist_ok as True to implicitly create directories when nonexistent.
• Instead of relying on KeyErrors when accessing dictionary values, the get method is used to supply a default value.
• When a key might exist in one dictionary, the get method (which may return None) is coupled with the nullish or operator to use the value that is not None.
• Uses predefined main_source attribute as argument to the _compile_sass function.
• Uses the walrus operator to scope a template match to the condition when it is not None.
• Removes unnecessary implicit .close()s on file handles when using context managers.

Testing

• I have tested my changes and verified that they work as expected

How to test the changes

Just run ./local.sh

[github-actions]

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[tfuxu]

Thanks for this refactoring job @lavafroth! Overall looks good, but I'll do a proper review tomorrow

[tfuxu]

Alright, I've tested everything related to this refactor and it works flawlessly.

Thanks again for this refactor, I didn't thought anyone would be able to decipher this mess I wrote here 😅️

[tfuxu]

@0xMRTT Any objections, or can we merge this?

[0xMRTT]

LGTM We can merge ! Thanks for clearing the mess lol