Make PromptBuilder::isSupported public

[felixarntz]

Fixes #128

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: JasonTheAdams <jason_the_adams@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[JasonTheAdams]

@felixarntz Looks good! Just one suggestion: Now that this method is public, let's add unit tests for it.

[felixarntz]

@JasonTheAdams Tests added!

I've also refreshed this with latest trunk, and bumped the @since n.e.x.t annotations to say @since 0.3.0. So we should be able to ship shortly after this is merged, pending some smoke testing using cli.php.

[JasonTheAdams]

Looks great! 🚀