Add support for modifiers on prompts

[tbckr]

The subcommands code and shell are text based completion requests that automatically get a modifier as a system completion message attached.

Add support for additional, custom modifiers, which can be referenced via a flag on the txt subcommand.

TBD: Maybe, we want to save the existing modifiers (code, shell) in config files as well. Then, code and shell would be shorthands for sgpt specific modifiers. The shell subcommand inherit this in addition to the execution functionality.

[tbckr]

• save modifiers in yaml file
• example file is prompts.yml in the root of the repo
• a modifier is a collection of prompts / chat messages
• save prompts.yml in the application specific config folder
• read modifiers from this config file
• use go templating engine to pass in variables (shell environment, OS, ...)
• subcommand to convert existing chat to modifier
• overwrite existing modifiers (shell, code) with this config file

[tbckr]

Additionally: Replace existing templating for shell command with go templating engine. Use templating and pass in environment specific variables (like shell environment, OS, ...) to further refine existing prompts/modifiers.