Add cursor shape configuration for each edit mode (#7745)

# Description

This PR allows the configuration of cursor shapes in nushell for each
edit mode. This is the change that is in the default_config.nu file.
```
  cursor_shape: {
    emacs: line # block, underscore, line (line is the default)
    vi_insert: block # block, underscore, line (block is the default)
    vi_normal: underscore # block, underscore, line  (underscore is the default)
  }
```

# User-Facing Changes

See above. If you'd prefer a different default, please speak up and let
us know.

# 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-utils/src/sample_config/default_config.nu

@@ -395,6 +395,11 @@ let-env config = {
     metric: true # true => KB, MB, GB (ISO standard), false => KiB, MiB, GiB (Windows standard)
     format: "auto" # b, kb, kib, mb, mib, gb, gib, tb, tib, pb, pib, eb, eib, zb, zib, auto
   }
+  cursor_shape: {
+    emacs: line # block, underscore, line (line is the default)
+    vi_insert: block # block, underscore, line (block is the default)
+    vi_normal: underscore # block, underscore, line  (underscore is the default)
+  }
   color_config: $dark_theme   # if you want a light theme, replace `$dark_theme` to `$light_theme`
   use_grid_icons: true
   footer_mode: "25" # always, never, number_of_rows, auto