`fish_config theme`:
- `list` to list all available themes (files in the two theme
directories - either the web_config/themes one or
~/.config/fish/themes!)
- `show` to show select (or all) themes right in the terminal - this
starts another fish that reads the theme file and prints the sample
text, manually colored
- `choose` to load a theme *now*, setting the variables globally
- `save` to load a theme and save the variables universally
- `dump` to write the current theme in .theme format (to stdout)
- `demo` to display the current theme
The file is called "config.fish", not "init.fish". We'll call it
"configuration" now.
"Initialization" might be slightly more precise, but in an irritating
way.
Also some wording improvements to the section. In particular we now
mention config.fish *early*, before the whole shebang.
It's a bit weird to *have* to fire up a browser to get fish_config to
choose a prompt.
So this adds a `prompt` subcommand to `fish_config`:
- `fish_config prompt list` shows all the available prompt names
- `fish_config prompt show` demos the available sample prompts
- `fish_config prompt choose` sources a prompt
- `fish_config prompt save` makes the choice permanent
A bare `fish_config` or `fish_config browse` opens the web UI.
Part of #3625.
TODO: This shows the right prompt on a new line. Showing it in-line is awkward
to do because we'd have to move it to the right.
