Selectors API: Rename Duotone parent from filters to filter

[aaronrobertshaw]

Related:

• Migrate duotone block support attributes under filter instead of color #49340
• Utilize the new selectors API for duotone #49342
• Block.json: Refactor and stabilize selectors API #46496
• Duotone: Update to use selectors API #49325

What?

Moves duotone selectors under filter instead of the plural filters.

Why?

This is to help bring consistency between all the various locations for duotone e.g. theme.json, block supports, selectors etc. See #49340 for further context.

How?

• Renames filters to filter
• Updates the global function to retrieve block CSS selectors to request filter.duotone
• Updates the tests for the global function
• Updates theme.json class to request filter.duotone

Testing Instructions

1. Ensure selectors tests pass: npm run test:unit:php -- --filter WP_Get_Block_CSS_Selector_Test
2. To test manually you can check out Duotone: Update to use selectors API #49325 which will be updated to use filter.duotone as well and be based on this PR.

[aaronrobertshaw]

Closing as these changes were absorbed into a single PR cleaning up the Selectors API (#49393).