Pluginval as dependency

[sudara]

Opening as a draft for feedback and iteration.

Addresses #137

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

• Moves JUCE from submodule to CPM dependency on 8.0.4
• 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• Removes PLUGINVAL_FETCH_JUCE option (no longer needed, superseded by not being top level)

Open Questions:

• Do we need to enforce minimum JUCE version for this use case?

---

Note to Dave and other maintainers:

With CPM, it's always a good idea to have the environment variable CPM_SOURCE_CACHE set — this will let you do offline builds vs. hitting the network on reconfigure. We can also add this optimization in CI too.

Add the following to .bashrc or .zshrc:

export CPM_SOURCE_CACHE=$HOME/.cache/CPM

The submodule is gone, so you can optionally clean up your .git/config folder, you can run:

rm -rf .git/modules/modules/juce