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-command/src/viewers/explore.rs

@@ -71,7 +71,7 @@ impl Command for Explore {
 
         let config = engine_state.get_config();
         let color_hm = get_color_config(config);
-        let style = theme_from_config(&config.explore_config);
+        let style = theme_from_config(&config.explore);
 
         let view_cfg = ViewConfig::new(config, &color_hm, &style);