feat: Add simple theme plugin to customise simple theme

Description

Adds a new grove-simple-theme plugin that can be used to generate build and install simple theme tokens.

Private-ref: https://tasks.opencraft.com/browse/BB-7833

Testing instructions

Steps to test the changes:

1. Install this plugin in a tutor environment.
2. Customize the theme using the GROVE_THEMES setting. You can use the example given later.
3. Run tutor images build mfe and deploy or start the tutor instance.
4. The MFE UI should be themed.

Example config:

GROVE_THEMES:
  core:
    size.border.radius.base: 0
    size.border.radius.lg: 0
    size.border.radius.sm: 0
  light:
    colors:
      brand: '#ff0080'
      primary: '#00ffaa'

The above will make all the buttons and UI elements square instead of rounded and apply a pink branding theme with a bright green primary color as can be seen in the screenshot below.

Dependencies

Designed to work better with: https://github.com/overhangio/tutor/pull/1302

Screenshots

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• Git commit history is clean
• Git commits are following conventional-commit practices
• Unit tests are added/updated
• Documentation is added/updated