chore: add issue and pull request templates

[Levdbas]

Issue

The repository did not provide structured templates for bug reports, feature requests, or pull requests.

Solution

Add GitHub health files based on Timber's templates, adapted for ACF Builder:

• A bug-report issue form that collects a minimal reproduction and relevant ACF Builder, ACF, WordPress, and PHP versions.
• A feature-request template that prompts for the problem, proposed API or behavior, alternatives, and supporting context.
• A pull-request template that captures the issue, solution, impact, usage changes, and testing.

Impact

No changes to the library's public API, generated ACF configuration, or runtime behavior.

Usage changes

None.