Rename $env.config.explore_config to $env.config.explore (for consistency with $env.config.ls, $env.config.table etc.) (#7317)

# Description

* `$env.config.explore_config` renamed to `$env.config.explore`. This
follows the principle that config subrecords relating to single commands
(as this relates to `explore`) should be exactly named after the command
(see `ls`, `rm`, `table` etc.)
* In `into_config()`, moved the match arm relating to
`$env.config.explore` out of the "legacy options" section (which is
slated for removal in a later version).

# User-Facing Changes

`explore` is not in any public releases yet, so this has no end-user
impact.

# 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

@@ -262,6 +262,20 @@ let-env config = {
       truncating_suffix: "..." # A suffix used by the 'truncating' methodology
     }
   }
+  explore: {
+    highlight: { bg: 'yellow', fg: 'black' }
+    status_bar: { bg: '#C4C9C6', fg: '#1D1F21' }
+    command_bar: { fg: '#C4C9C6' }
+    split_line: '#404040'
+    cursor: true
+    # selected_column: 'blue'
+    # selected_row: { fg: 'yellow', bg: '#C1C2A3' }
+    # selected_cell: { fg: 'white', bg: '#777777' }
+    # line_shift: false,
+    # line_index: false,
+    # line_head_top: false,
+    # line_head_bottom: false,
+  }
   history: {
     max_size: 10000 # Session has to be reloaded for this to take effect
     sync_on_enter: true # Enable to share history between multiple sessions, else you have to close the session to write history to file
@@ -293,22 +307,6 @@ let-env config = {
   show_banner: true # true or false to enable or disable the banner
   render_right_prompt_on_last_line: false # true or false to enable or disable right prompt to be rendered on last line of the prompt.
 
-  # A 'explore' utility config
-  explore_config: {
-    highlight: { bg: 'yellow', fg: 'black' }
-    status_bar: { bg: '#C4C9C6', fg: '#1D1F21' }
-    command_bar: { fg: '#C4C9C6' }
-    split_line: '#404040'
-    cursor: true
-    # selected_column: 'blue'
-    # selected_row: { fg: 'yellow', bg: '#C1C2A3' }
-    # selected_cell: { fg: 'white', bg: '#777777' }
-    # line_shift: false,
-    # line_index: false,
-    # line_head_top: false,
-    # line_head_bottom: false,
-  }
-
   hooks: {
     pre_prompt: [{
       $nothing  # replace with source code to run before the prompt is shown