Fix linter errors when running strict checks

[inzyster]

Background

I've been using this library in my projects, and I always run linting checks like so:
odin check . -vet -strict-style -disallow-do -warnings-as-errors as both a pre-commit hook and in my CI workflows. As a result, the linter raised a few diagnostics in this library's code.

Solution

• Removed the import from afmt_posix.odin
• Replaced the name variable shadowing in print_color_guide and print_color_guide_ex in colors.odin with assignment
• Replaced the color variable shadowing in rgb_to_8bit in afmt.odin with a new name (_color)

Validation

To validate the changes, run the linter:
odin check . -vet -strict-style -disallow-do -warnings-as-errors -no-entry-point