Add on and this Toolshed Commands

[ishkab]

About the PR

This PR adds two Toolshed commands (8 total subcommands) for filtering entities based on map or grid predicates. All six subcommands of on are invertible with the usual not construction. These are mainly convenience functions for administration.

• on:grid <gridid> filters an IEnumerable to only those entities that are on the grid with the ID given.
• on:thisgrid filters an IEnumerable to only those entities that are on the same grid as the invoker.
• on:gridof <ent> filters an IEnumerable to only those entities that are on the same grid as the entity given.
• on:map <mapid> filters an IEnumerable to only those entities that are on the map with the ID given.
• on:thismap filters an IEnumerable to only those entities that are on the same map as the invoker.
• on:mapof <ent> filters an IEnumerable to only those entities that are on the same map as the entity given.
• this:grid returns the grid that the invoker is on.
• this:map returns the ID of the map that the invoker is on.

Why / Balance

• These are Toolshed filters that I personally want.
• They may be generally useful to anyone who uses Toolshed, e.g. in execs. With on:thisgrid, one can easily target only entities on-station, for instance.
• Mappers should find the on command useful for removing stray grids created in error during the mapping process, as in entities on:thismap with MapGrid not with BecomesStation delete.

Technical details

I originally implemented this way out of code style with the rest of the repo. I think the code is less readable now, but it conforms.

I added OnCommand and ThisCommand along with three Toolshed errors. The commands require DEBUG permission.

Requirements

• I have read and am following the Pull Request and Changelog Guidelines.
• I have added media to this PR or it does not require an ingame showcase.

Breaking changes

I tested this very briefly and found it to be in enough of a working order to PR. If bugs are discovered, they should be strictly contained to the execution of the commands - a situation which makes for easy avoidance until a patch can be issued.