Overhaul explore config (#13075)

Configuration in `explore` has always been confusing to me. This PR
overhauls (and simplifies, I think) how configuration is done.

# Details

1. Configuration is now strongly typed. In `Explore::run()` we create an
`ExploreConfig` struct from the more general Nu configuration and
arguments to `explore`, then pass that struct to other parts of
`explore` that need configuration. IMO this is a lot easier to reason
about and trace than the previous approach of creating a
`HashMap<String, Value>` and then using that to make various structs
elsewhere.
2. We now inherit more configuration from the config used for regular Nu
tables
1. Border/line styling now uses the `separator` style used for regular
Nu tables, the special `explore.split_line` config point has been
retired.
2. Cell padding in tables is now controlled by `table.padding` instead
of the undocumented `column_padding_left`/`column_padding_right` config
3. The (optional, previously not enabled by default) `selected_row` and
`selected_column` configuration has been removed. We now only highlight
the selected cell. I could re-add this if people really like the feature
but I'm guessing nobody uses it.

The interface still looks the same with a default/empty config.nu:


![image](https://github.com/nushell/nushell/assets/26268125/e40161ba-a8ec-407a-932d-5ece6f4dc616)

crates/nu-utils/src/sample_config/default_config.nu

@@ -189,12 +189,7 @@ $env.config = {
             warn: {}
             info: {}
         },
-        table: {
-            split_line: { fg: "#404040" },
-            selected_cell: { bg: light_blue },
-            selected_row: {},
-            selected_column: {},
-        },
+        selected_cell: { bg: light_blue },
     }
 
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