Extract one-key-formatting into own feature

[fregante]

This feature (#2145) was implemented as part of comment-fields-keyboard-shortcuts but it would be easier to find if it was in its own file.

#2145 (comment)

PR welcome to extract this into its own feature probably named one-key-formatting

cc @notlmn

[fregante]

Alternatively, indentTextarea could come together in the same feature and be called something like textarea-editor-shortcuts, to make it sound like it's a feature that adds text editor-like behavior to large text fields.