feat: Add validation against passing both color and foreground to InlineTextStyle

[luanpotter]

Description

Add validation against passing both color and foreground to InlineTextStyle.

Note: I plan on working on standardizing asserts, exceptions and tests thereof on a followup, so I am not focusing on this particular issue here.

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [ ] Yes, this PR is a breaking change.
• [x] No, this PR is not a breaking change.