JUCE as a submodule

The cmake addition is great (thanks @showlabor)!
I'd like to suggest making JUCE a submodule though.

The way it is included via CPM now means it downloads around 250Mb
every time a new build folder is created or JUCE gets updated.
Adding it as a submodule with depth1 you end up with aprox 60Mb once,
updates are incremental and it's independent from the build folder.

This will also speed up potential pipeline builds.

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "libs/surgesynthteam_tuningui"]
 	path = libs/surgesynthteam_tuningui
 	url = https://github.com/surge-synthesizer/surgesynthteam_tuningui.git
+[submodule "libs/JUCE"]
+	path = libs/JUCE
+	url = https://github.com/juce-framework/JUCE.git