feat: make primary and error color configurable

[javaes]

I'm quite new to rust, so posting this as a draft first, please check if I'm on the right track.

Things I still plan to do:

• fallback to default if color config is unparsable
• add tests
• add documentation
• add changelog entry

Description

Making the primary and the error colors configurable.

Closes #193

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

QA

How did you test this?

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked

[LucasPickering]

Hey @javaes thanks for this, unfortunately I started working on this last night and also have something almost done :( I just put my up in another PR if you want to reference it; Ours look very nearly identical, the main difference is mine stores the colors as the Color type from Ratatui, which means 1. we never have to copy colors and 2. It does all the deserialization for us. The one thing remaining is I want to replace the contrast_text function with just an additional field in the theme called primary_color_contrast, which will let the user select which color to use for text with the primary color background (generally will be white or black). I'll be doing that tomorrow probably since I won't have time today.

[javaes]

No worries, anyways still a learning experience for me! 👍