Enhancement: Command to identify if a particular assembler is in use

[TobyLobster]

Instead of the proposed annotate_beebasm(), have a command to check the assembler in use, e.g.

if is_assembler("beebasm"):
    annotate(...)

This seems more flexible. This could warn the user if the name of the assembler given is not known to py8dis, and a output a list of all known assembler names.

[TobyLobster]

Implemented.