Load types and run types

[stevesims]

on RISC OS you could have system variables with names starting Alias$@LoadType_ and Alias$@RunType_ following which was the 12-bit filetype code in hex. this would define the command to use when a file of that given filetype was either "loaded" or "run"

it would be good to implement essentially the same mechanism, but using filename extensions. this would let us, for instance, be able to directly run a BASIC program from the MOS command prompt just by typing that program's filename

(this of course builds on #54 )

[stevesims]

runtypes are planned - it should be possible to run a file using RunFile <filename> [<args>]

I'm not sure about the inclusion of load types - they're useful on RISC OS, but not so much on Agon MOS. if they do get added then a companion LoadFile command would be used which would work like RunFile

[stevesims]

both LoadFile and RunFile now exist in the MOS 3 branch

so far the CLI interpreter has not been updated to use RunFile - some issues remain in working out exactly how that should be done. how the CLI interpreter changes will also have implications for how tab-completion works