[enhancement] Improve organization & behavior of model merging and textual inversion scripts

[lstein]

• Model merging and textual inversion scripts have been moved into ldm/invoke, which allows them to be installed properly by pyproject.toml.
• As part of the pyproject install, the .py suffix is removed from the command. I.e. use invoke, configure_invokeai, merge_models and textual_inversion.
• GUI versions are activated by adding --gui to the command. Without this, you get a classical argv-based command. Example: merge_models --gui
• Fixed up the launcher scripts to accommodate new naming scheme.
• Keyboard behavior of the GUI front ends has been improved. You can now use up and down arrow to move from field to field, in addition to and ctrl-N/ctrl-P

[mauwii]

something is strange:

previously my Apple M1 was used like 90-95% while running invoke, this is not the case right now 🤔

Hmmmm, switched from sd-1.5 to sd-2.1, which is working fine, so maybe just my odd configuration...