Add Linux support

[nick-thompson]

Linux support is already nearly there, I just need a bit of help getting it over the finish line from anyone who has more experience with Linux VST3.

The work involved is as follows:

• Inspect the VST3 bundle layout on Linux and update the getAssetsDirectory() here for the JUCE_LINUX case
• Attach the native webview handle to the viewContainer for the JUCE_LINUX case here and here
• Re-enable CI running on linux here and update the required dependency install step here so that the builds are happy

I think that should be it, but it's very possible I'm missing something.

[erik-nilcoast]

I have most of this done in #11

[erik-nilcoast]

I'm going to dig more into the API for attaching an X11 window. I suspect I have to find the existing window and attach that handle?

[nick-thompson]

Thank you so much! I left a comment to that question on your PR