[Theming] Separate text colours for surface and surface containers

### Feature Description

- Adding the values "surfaceContainerText", "surfaceContainerHighText", and "surfaceContainerHighestText" to the colour schemes.
- Adding the corresponding "surfaceVariantText" values.

### Use Case

Right now, "surface", "surfaceContainer", "surfaceContainerHigh", and "surfaceContainerHighest" all use the same "surfaceText" colour. This makes it impossible to make themes with contrasting colours (e.g. black and white), because the text will be illegible or invisible on one of those.
Same applies to "surfaceVariant" and "surfaceVariantText". Additionally, "surfaceVariantText" is currently being used also over "surface", not just over "surfaceVariant".
Having separate values would fix this.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

To preserve the functionality of existing themes, "surfaceText" and "surfaceVariantText" could be set as default or fallback when the other values are missing.

### Alternatives/Existing Solutions

_No response_