Pluginval as dependency

[sudara]

Closes #137

This improves support for pluginval as a CMake sub-project in a JUCE project.

• Moves JUCE from submodule to CPM dependency on 8.0.11
• Only adds JUCE when top level
• Only sets deployment target when top level
• Only generate docs when top level
• Removes PLUGINVAL_FETCH_JUCE option (no longer needed, superseded by not being top level)
• Adds documentation
• Adds some conditionals around 8.0.11 deprecations to allow earlier versions of JUCE.

Caveats:

Technically, this PR would make maintenance more complicated — it's now BYOJ, Bring Your Own JUCE — which means pluginval will be compiled against more than one version of JUCE. We could add a caveat in the README that there's no guarantees on other JUCE versions than what pluginval depends on, specify that it's JUCE 8 only, etc...

[drowaudio]

Am I supposed to be reviewing or merging this?
I like the feature set but it seems to have not changed much in the last year? Is it stable enough to go in?

You might notice I switched to CPM for a few other dependancies on develop. Can this branch be pulled in line with that?

[sudara]

@drowaudio I've been using this branch for a year as a dependency of my synth but never got the example plugins running in CI. I'll need to get that going and resolve recent conflicts before it's mergable.

[drowaudio]

Ah ok. Poke me if/when you get to that.

I might switch to juce via CPM at some point soon anyway...