Add support for more search form fields in Instant Results

[burhandodhy]

Description of the Change

This PR adds support for more search form fields when opening Instant Results from a submitted form. Values are picked up automatically when their field names match one of these rules:

• Explicit map (filterable): The field name is mapped to an Instant Results arg via the ep.instantResults.queryVarMap filter.
• Direct schema match: The field name matches a key in the Instant Results args schema (e.g. post_type).
• Taxonomy pattern: The field name matches a taxonomy slug and resolves to a tax-{taxonomy} arg that exists in the schema.

Closes #2978

How to test the Change

Changelog Entry

Added - Add support for more search form fields in Instant Results

Credits

Props @burhandodhy @JakePT

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.