Also use branches named main in workflow

Since GitHub defaults to 'main' now, also include it in the workflow
when building, so developers don't have to change branch names
if they choose to use it.

.github/workflows/main.yml

@@ -6,6 +6,7 @@ on:
       - '**.md'
     branches:
       - master
+      - main
     tags:
       - '*'
   pull_request:
@@ -13,6 +14,7 @@ on:
       - '**.md'
     branches:
       - master
+      - main
 
 env:
   PLUGIN_NAME: 'obs-plugintemplate'