UX improvements to the custom command workflow

[toolmantim]

UX improvements to the custom command workflow:

• Improved labels
• Gives you the choice of where to store the custom command
• Saved notification now has a button for opening the settings and inspecting the results
• Fixes bug where you can incorrectly choose the same slash name as an existing command
• Move examples from the generated config to the new Custom Commands docs

Requires

• Add custom Cody commands documentation sourcegraph#56489

Test plan

• Opened the custom commands quickpick
• Manually ran through the new command builder flow
• Clicked new buttons, inspected JSON

[dominiccooney]

This looks good to me.

It would be better if we added a type like:

type CustomPromptType = 'user' | 'workspace'

and referred to that everywhere instead of repeating 'user' | 'workspace'. That will considerably reduce toil if we add a third kind of custom command, maybe ones synced with your whole Enterprise or something, because we can expand the type in one place and follow the type errors.

[toolmantim]

@dominiccooney thanks! Updated to use CustomCommandType in 1c96984.