Customizable/configurable status line (#2434)

* feat(statusline): add the file type (language id) to the status line

* refactor(statusline): move the statusline implementation into an own struct

* refactor(statusline): split the statusline implementation into different functions

* refactor(statusline): Append elements using a consistent API

This is a preparation for the configurability which is about to be
implemented.

* refactor(statusline): implement render_diagnostics()

This avoid cluttering the render() function and will simplify
configurability.

* feat(statusline): make the status line configurable

* refactor(statusline): make clippy happy

* refactor(statusline): avoid intermediate StatusLineObject

Use a more functional approach to obtain render functions and write to
the buffers, and avoid an intermediate StatusLineElement object.

* fix(statusline): avoid rendering the left elements twice

* refactor(statusline): make clippy happy again

* refactor(statusline): rename `buffer` into `parts`

* refactor(statusline): ensure the match is exhaustive

* fix(statusline): avoid an overflow when calculating the maximal center width

* chore(statusline): Describe the statusline configurability in the book

* chore(statusline): Correct and add documentation

* refactor(statusline): refactor some code following the code review

Avoid very small helper functions for the diagnositcs and inline them
instead.
Rename the config field `status_line` to `statusline` to remain
consistent with `bufferline`.

* chore(statusline): adjust documentation following the config field refactoring

* revert(statusline): revert regression introduced by c0a1870

* chore(statusline): slight adjustment in the configuration documentation

* feat(statusline): integrate changes from #2676 after rebasing

* refactor(statusline): remove the StatusLine struct

Because none of the functions need `Self` and all of them are in an own
file, there is no explicit need for the struct.

* fix(statusline): restore the configurability of color modes

The configuration was ignored after reintegrating the changes of #2676
in 8d28f95.

* fix(statusline): remove the spinner padding

* refactor(statusline): remove unnecessary format!()

1 files changed, 31 insertions, 1 deletions

diff --git a/book/src/configuration.md b/book/src/configuration.md
index 0a6e5fdd..4c849f26 100644
--- a/book/src/configuration.md
+++ b/book/src/configuration.md
@@ -48,13 +48,43 @@ hidden = false
 | `rulers` | List of column positions at which to display the rulers. Can be overridden by language specific `rulers` in `languages.toml` file. | `[]` |
 | `color-modes` | Whether to color the mode indicator with different colors depending on the mode itself | `false` |
 
+### `[editor.statusline]` Section
+
+Allows configuring the statusline at the bottom of the editor.
+
+The configuration distinguishes between three areas of the status line:
+
+`[ ... ... LEFT ... ... | ... ... ... ... CENTER ... ... ... ... | ... ... RIGHT ... ... ]`
+
+Statusline elements can be defined as follows:
+
+```toml
+[editor.statusline]
+left = ["mode", "spinner"]
+center = ["file-name"]
+right = ["diagnostics", "selections", "position", "file-encoding", "file-type"]
+```
+
+The following elements can be configured:
+
+| Key    | Description |
+| ------ | ----------- |
+| `mode` | The current editor mode (`NOR`/`INS`/`SEL`) |
+| `spinner` | A progress spinner indicating LSP activity |
+| `file-name` | The path/name of the opened file |
+| `file-encoding` | The encoding of the opened file if it differs from UTF-8 |
+| `file-type` | The type of the opened file |
+| `diagnostics` | The number of warnings and/or errors |
+| `selections` | The number of active selections |
+| `position` | The cursor position |
+
 ### `[editor.lsp]` Section
 
 | Key                | Description                                 | Default |
 | ---                | -----------                                 | ------- |
 | `display-messages` | Display LSP progress messages below statusline[^1] | `false` |
 
-[^1]: A progress spinner is always shown in the statusline beside the file path.
+[^1]: By default, a progress spinner is shown in the statusline beside the file path.
 
 ### `[editor.cursor-shape]` Section