Customizable/configurable status line (helix-editor#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 helix-editor#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 helix-editor#2676
in 8d28f95.

* fix(statusline): remove the spinner padding

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

helix-term/src/ui/editor.rs

@@ -16,7 +16,7 @@ use helix_core::{
     LineEnding, Position, Range, Selection, Transaction,
 };
 use helix_view::{
-    document::{Mode, SCRATCH_BUFFER_NAME},
+    document::Mode,
     editor::{CompleteAction, CursorShapeConfig},
     graphics::{Color, CursorKind, Modifier, Rect, Style},
     input::{KeyEvent, MouseButton, MouseEvent, MouseEventKind},