add --mime-type(-m) to ls in the type column (#7616)

# Description

This PR adds the `mime-type` to the `type` column if you add the
`--mime-type(-m)` flag to `ls`.
<img width="853" alt="Screenshot 2022-12-27 at 11 43 20 AM"
src="https://user-images.githubusercontent.com/343840/209705499-27fe40fe-0356-4d9d-97f2-4b2dc52e0963.png">

<img width="781" alt="Screenshot 2022-12-27 at 11 45 53 AM"
src="https://user-images.githubusercontent.com/343840/209705509-4d677389-fd68-401e-a7af-3fc6052743b6.png">

# User-Facing Changes

If you specify the `-m` flag, you get the "guessed at" mime type. The
guess is based on the file name and uses this crate
https://docs.rs/mime_guess/latest/mime_guess/ for the guessing.

Part of issue #7612 and and #7524

There's some debate on if the `mime-type` should be added to the `type`
column or if there should be a separate `mime` column. I tend to lean on
the side of `type` since it's technically a type and it's only in that
column if you ask it to be there. Also, I'd prefer to reuse a column
rather than having a list of sprawling columns. Also, as @KodiCraft
suggested, there is precedence as with `ls -d` where the summed size is
in the size column.

I could go either way and if someone wants to create a `mime` column,
we'd probably accept it.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-cli/tests/completions.rs

@@ -459,7 +459,7 @@ fn flag_completions() {
     // Test completions for the 'ls' flags
     let suggestions = completer.complete("ls -", 4);
 
-    assert_eq!(14, suggestions.len());
+    assert_eq!(16, suggestions.len());
 
     let expected: Vec<String> = vec![
         "--all".into(),
@@ -468,6 +468,7 @@ fn flag_completions() {
         "--full-paths".into(),
         "--help".into(),
         "--long".into(),
+        "--mime-type".into(),
         "--short-names".into(),
         "-D".into(),
         "-a".into(),
@@ -475,6 +476,7 @@ fn flag_completions() {
         "-f".into(),
         "-h".into(),
         "-l".into(),
+        "-m".into(),
         "-s".into(),
     ];