initial implementation of bufferline (helix-editor#2759)

* initial implementation of bufferline

* fixed lint

* changed to 'bufferline', added enum for config modes, some cleanup

* fixed lint

* added file modification indicator

* removed redundant code, added proper themeing with fallback, changed 'file modified' indicator

* remove commented code

* Update helix-term/src/ui/editor.rs

simplify text and offset computation

Co-authored-by: Gokul Soumya <gokulps15@gmail.com>

* add ui.bufferline.background key for themes

Co-authored-by: lazytanuki <43273245+lazytanuki@users.noreply.github.com>

* address PR comments

* Update helix-term/src/ui/editor.rs

* simplify computation of editor area:

* change to set_stringn to avoid overflow

* Update configuration.md

Updates documentation to reflect decision re: defaulting to never showing bufferline.

* addressed pr comments

* fix build error

* address pr comments

* revert accidental change

Co-authored-by: Gokul Soumya <gokulps15@gmail.com>
Co-authored-by: lazytanuki <43273245+lazytanuki@users.noreply.github.com>
Co-authored-by: Seth Bromberger <sbromberger@users.noreply.github.com>

helix-term/src/ui/editor.rs

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