Add submodule Assets/Plugins/LSL4Unity

Dependencies localisation on Assets/Plugins is a standard.

.gitmodules

-[submodule "Assets/LSL4Unity"]
-	path = Assets/LSL4Unity
+[submodule "Assets/Plugins/LSL4Unity"]
+	path = Assets/Plugins/LSL4Unity
 	url = https://github.com/tmonseigne/LSL4Unity.git

Assets/Plugins.meta

+fileFormatVersion: 2
+guid: 21979a78add3606478b948018369317c
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

LSL4Unity @ 5379c1a3

+Subproject commit 5379c1a3fc9923bc882e4f75a8a8aa55a46fea21

Assets/Plugins/LSL4Unity.meta

+fileFormatVersion: 2
+guid: 60cccd48c61b7584d84ae9ad35c1a949
+folderAsset: yes
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: