Set "as" attribute on stylesheet link tag

[iainbeeston]

Description

In the browser javascript console I see this error on every page load:

The resource http://localhost:3000/avo-assets/avo.base.css was preloaded using link preload but not used within a few seconds from the window's load event. Please make sure it has an appropriate `as` value and it is preloaded intentionally."

It's not causing me any issues as such - it's just noisy and can make it harder to see real issues. It seems like a simple thing to fix so I've raised a PR.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. The avo dashboard should load with the correct styles and show no stylesheet warning in the browser console.

Manual reviewer: please leave a comment with output from the test if that's the case.

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[adrianthedev]

I didn't know about the as attribute