Add missing REIs

[Ben-Davis1]

We need to add any missing REIs throughout the project. This is especially important for places where we have fixed the duplication of bs.as (see #21) as some modules shouldn't be available for use outside of the module they're defined in.

It would be good to make a list of needed REIs in this issue so we can work on it

[tsnobip]

I think the missing .rei files are:

• PM_ChangeSet.rei (implementation file added in BS7 compatibility (fix bs.as duplicates with unboxed variants) #21 )
• PM_Command.rei
• PM_Decoration.rei
• PM_EditorProps.rei
• PM_NodeView.rei
• PM_SchemaBasic.rei
• PM_PM_SchemaList.rei
• PM_View.rei
• PMPlugins_DropCursor.rei
• PMPlugins_History.rei
• PMPlugins_InputRules.rei
• PMPlugins_Keymap.rei

What might be more cumbersome and error-prone is changing the existing .rei files to use external instead of let and add all the bs annotations.