Allow marking certain standalone plugins as experimental

[felixarntz]

Follow up to #1031: Similar to how modules can be marked as experimental, we should maintain the ability to have certain standalone plugins marked as experimental in the Performance Lab UI. This indicates that a certain plugin may be in an earlier phase and is just starting out, or it contains "riskier" features that we are for instance less confident about being used in production. While Performance Lab as a whole is sort of a Beta plugin, some of our standalone plugins are "more beta than others".

As with modules, of course any plugin can and should eventually graduate to non-experimental.

The primary reason to continue indicating and supporting the distinction between experimental and non-experimental modules, other than end user awareness, is the onboarding flow (see #1032). One of its requirements is to have a one-click button to activate all "recommended" features - which effectively means activating all "non-experimental" features.

For this issue, let's implement the following (after #1031 is done):

• Additionally to the standalone plugins' slug and version constants hard-coded in the Performance Lab codebase, each standalone plugin should receive a boolean indicator for whether it's experimental or not.
• Of the current 5 standalone plugins, only the auto-sizes plugin should be marked as experimental. This is mostly because there's currently no browser where its functionality is actually supported (given Chrome recently rolled back the feature).
• The feature cards for the plugins on the PL settings screen should show a note "experimental" for those plugins which are experimental. Experimental plugins should be displayed after the non-experimental ones.

[felixarntz]

@joemcgill @mukeshpanchal27 LMKWYT on the description and proposed requirements.

[felixarntz]

Fixed via #1111