CSS: New var proposal + init headings-color

[louismaximepiton]

Description

Create a new --bs-link-opacity variable.
Change $headings-color from null to inherit.

Motivation & Context

I can't see any breaking change inside those changes but let me know if you spot one !

Harmonize the use of CSS var and fix a weird case in colored links.
Remove a weird behavior of --bs-heading-color which was the only one where we check the initialization. IMO, it could be implemented in Bootstrap but on all the CSS variables.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-37904--twbs-bootstrap.netlify.app/
• https://deploy-preview-37904--twbs-bootstrap.netlify.app/docs/5.3/helpers/colored-links

Related issues

[XhmikosR]

FYI the CI checks fail for a reason 😛

Please don't merge blindly.