Add --stdin-filepath option to format cmd

[lffg]

This PR introduces the --stdin-filepath option to the format command.

This was inspired by ESLint's --stdin-filename and Prettier's --stdin-filepath. It doesn't have any behavior in the actual parsing of files or their resolution.

It is mainly useful when showing error information. In such cases, instead of getting:

  ┌─ -:2:5
  │
2 │ [bar=6
  │     ^ expected "]"

One would get:

  ┌─ ./foo.toml:2:5
  │
2 │ [bar=6
  │     ^ expected "]"

By executing format as taplo format --stdin-filepath ./foo.toml -.

[tamasfe]

Thanks, I left a comment about a tiny detail, otherwise LGTM.