feature: Beginnings of in-app config

[ClementTsang]

Description

A description of the change and what it does. If relevant, please provide screenshots of what results from the change:

Initial refactorings and additions to support in-app config.

This PR only does a few things, in order to not make giant PRs:

• Refactor our current options logic to support in-app configs. That is, we can write to a config file with our changes now.
• The default action when creating a new config file is to leave it blank. (TBD and for now, not sure on this one)
• Previously, we would set everything in a config file on startup; now we need to read from the config TOML struct whenever.
• C keybind is now occupied for configs.
• no_write option to never write to a config file.

Issue

If applicable, what issue does this address?

Part of #159. Closes #241

Type of change

Remove the irrelevant ones:

• Refactor

Test methodology

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Furthermore, please tick which platforms this change was tested on:

• Windows
• macOS
• Linux

Checklist

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• Change has been tested to work
• Areas your change affects have been linted using rustfmt
• Code has been self-reviewed
• Code has been tested and no new breakage is introduced unless intended
• Passes CI tests
• Documentation has been added/updated if needed (README, help menu, etc.)
• No merge conflicts arise from the change

Other information

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