Refactor and fix Config<->Value mechanism (#10896)

# Description
Our config exists both as a `Config` struct for internal consumption and
as a `Value`. The latter is exposed through `$env.config` and can be
both set and read.
Thus we have a complex bug-prone mechanism, that reads a `Value` and
then tries to plug anything where the value is unrepresentable in
`Config` with the correct state from `Config`.

The parsing involves therefore mutation of the `Value` in a nested
`Record` structure. Previously this was wholy done manually, with
indices.
To enable deletion for example, things had to be iterated over from the
back. Also things were indexed in a bunch of places. This was hard to
read and an invitation for bugs.

With #10876 we can now use `Record::retain_mut` to traverse the records,
modify anything that needs fixing, and drop invalid fields.

# Parts:

- Error messages now consistently use the correct spans pointing to the
problematic value and the paths displayed in some messages are also
aligned with the keys used for lookup.
- Reconstruction of values has been fixed for:
	- `table.padding`
	- `buffer_editor`
	- `hooks.command_not_found`
	- `datetime_format` (partial solution)
- Fix validation of `table.padding` input so value is not set (and
underflows `usize` causing `table` to run forever with negative values)
- New proper types for settings. Fully validated enums instead of
strings:
  - `config.edit_mode` -> `EditMode` 
  	- Don't fall back to vi-mode on invalid string
  - `config.table.mode` -> `TableMode`
- there is still a fall back to `rounded` if given an invalid
`TableMode` as argument to the `nu` binary
  - `config.completions.algorithm` -> `CompletionAlgorithm`
  - `config.error_style` -> `ErrorStyle`
    - don't implicitly fall back to `fancy` when given an invalid value.
- This should also shrink the size of `Config` as instead of 4x24 bytes
those fields now need only 4x1 bytes in `Config`
- Completely removed macros relying on the scope of `Value::into_config`
so we can break it up into smaller parts in the future.
- Factored everything into smaller files with the types and helpers for
particular topics.
- `NuCursorShape` now explicitly expresses the `Inherit` setting.
conversion to option only happens at the interface to `reedline`

crates/nu-cli/src/repl.rs

@@ -245,15 +245,9 @@ pub fn evaluate_repl(
 
         // Find the configured cursor shapes for each mode
         let cursor_config = CursorConfig {
-            vi_insert: config
-                .cursor_shape_vi_insert
-                .map(map_nucursorshape_to_cursorshape),
-            vi_normal: config
-                .cursor_shape_vi_normal
-                .map(map_nucursorshape_to_cursorshape),
-            emacs: config
-                .cursor_shape_emacs
-                .map(map_nucursorshape_to_cursorshape),
+            vi_insert: map_nucursorshape_to_cursorshape(config.cursor_shape_vi_insert),
+            vi_normal: map_nucursorshape_to_cursorshape(config.cursor_shape_vi_normal),
+            emacs: map_nucursorshape_to_cursorshape(config.cursor_shape_emacs),
         };
         perf(
             "get config/cursor config",
@@ -770,14 +764,15 @@ fn update_line_editor_history(
     Ok(line_editor)
 }
 
-fn map_nucursorshape_to_cursorshape(shape: NuCursorShape) -> SetCursorStyle {
+fn map_nucursorshape_to_cursorshape(shape: NuCursorShape) -> Option<SetCursorStyle> {
     match shape {
-        NuCursorShape::Block => SetCursorStyle::SteadyBlock,
-        NuCursorShape::UnderScore => SetCursorStyle::SteadyUnderScore,
-        NuCursorShape::Line => SetCursorStyle::SteadyBar,
-        NuCursorShape::BlinkBlock => SetCursorStyle::BlinkingBlock,
-        NuCursorShape::BlinkUnderScore => SetCursorStyle::BlinkingUnderScore,
-        NuCursorShape::BlinkLine => SetCursorStyle::BlinkingBar,
+        NuCursorShape::Block => Some(SetCursorStyle::SteadyBlock),
+        NuCursorShape::UnderScore => Some(SetCursorStyle::SteadyUnderScore),
+        NuCursorShape::Line => Some(SetCursorStyle::SteadyBar),
+        NuCursorShape::BlinkBlock => Some(SetCursorStyle::BlinkingBlock),
+        NuCursorShape::BlinkUnderScore => Some(SetCursorStyle::BlinkingUnderScore),
+        NuCursorShape::BlinkLine => Some(SetCursorStyle::BlinkingBar),
+        NuCursorShape::Inherit => None,
     }
 }