Fix asset loader bugs causing missing dependencies, lack of RTL support, lack of inlining for performance

[felixarntz]

What?

The Asset_Loader implementation has several problems:

• The PHP build asset check is wrong, it's currently always failing.
• RTL language support is missing.
• The stylesheet file path should be provided to benefit from conditional inlining CSS for performance in WP Core.
• It's not possible to pass asset data manually, which can lead to unperformant and sometimes inaccurate filemtime calls.

How?

This PR addresses the above issues.

Test using WordPress Playground

The changes in this pull request can be previewed and tested using this WordPress Playground instance:

Click here to test this pull request.

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Co-authored-by: felixarntz <[email protected]>
Co-authored-by: dkotter <[email protected]>

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