feat: cli command for listing backends

[roele]

Resolves #1983.

New Command
Adds a command backends with sub-command ls to list available backend types.

$ mise backends
cargo
go
npm
pipx
ubi

New Doctor Section
Adds a section backends to the doctor command output and filters out (installed) backends from the plugins section since these are shown in toolset.

...

backends: 
  cargo
  go
  npm
  pipx
  ubi

plugins: 
  bun         (core)
  deno        (core)
  erlang      (core)
...

[jdx]

that was quick!

[dhanak]

Please also add completions for the new command.

[roele]

@jdx on another note, i assume backends are still experimental should we mark the command as such?

[jdx]

well what we should probably do is just show "asdf" if experimental is not set