clean up config by removing legacy options (#9496)

# Description

This PR cleans up the deprecated legacy config options that were
deprecated in nushell version 0.72.0. These are the config points that
were in the root of the config but also duplicated in the nested
structures. For instance `use_ls_colors` was in the root of the config
and also in the `ls.use_ls_colors` nested structure. This was originally
done to preserve backwards compatibility when nested structures were
introduced in the config file.

Here's a list of the legacy config points that were removed.

- `use_ls_colors` - previously replaced with `ls.use_ls_colors`
- `rm_always_trash` - previously replaced with `rm.always_trash`
- `history_file_format` - previously replaced with `history.file_format`
- `sync_history_on_enter` - previously replaced with
`history.sync_on_enter`
- `max_history_size` - previously replaced with `history.max_size`
- `quick_completions` - previously replaced with `completions.quick`
- `partial_completions` - previously replaced with `completions.partial`
- `max_external_completion_results` - previously replaced with
`completions.external.max_results`
- `completion_algorithm` - previously replaced with
`completions.algorithm`
- `case_sensitive_completions` - previously replaced with
`completions.case_sensitive`
- `enable_external_completion` - previously replaced with
`completions.external.enable`
- `external_completer` - previously replaced with
`completions.external.completer`
- `table_mode` - previously replaced with `table.mode`
- `table_index_mode` - previously replaced with `table.index_mode`
- `table_trim` - previously replaced with `table.trim`
- `show_clickable_links_in_ls` - previously replaced with
`ls.clickable_links`
- `cd_with_abbreviations` - previously replaced with `cd.abbreviations`
- `filesize_metric` - previously replaced with `filesize.metric`
- `filesize_format` - previously replaced with `filesize.format`
- `cursor_shape_vi_insert` - previously replaced with
`cursor_shape.vi_insert`
- `cursor_shape_vi_normal` - previously replaced with
`cursor_shape.vi_normal`
- `cursor_shape_emacs` - previously replaced with `cursor_shape.emacs`

Removes log_level from the config since it doesn't do anything any
longer. We moved log-level to a nushell parameter some time ago.

Renames history_isolation to isolation in the config.nu for consistency.

Fixes a couple bugs where values weren't being set in the "//
Reconstruct" sections (history_isolation, table_show_empty).

Reorganized/Moved things around a tiny bit and added a few comments.

# User-Facing Changes

history.histor_isolation is now history.isolation.
If anyone is still using the legacy config points, deprecated since
0.72.0 2022-11-29, their config will break.

# Tests + Formatting
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crates/nu-utils/src/sample_config/default_config.nu

@@ -268,7 +268,7 @@ let-env config = {
     max_size: 100_000 # 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
     file_format: "plaintext" # "sqlite" or "plaintext"
-    history_isolation: true # true enables history isolation, false disables it. true will allow the history to be isolated to the current session. false will allow the history to be shared across all sessions.
+    isolation: true # true enables history isolation, false disables it. true will allow the history to be isolated to the current session. false will allow the history to be shared across all sessions.
   }
   completions: {
     case_sensitive: false # set to true to enable case-sensitive completions