LibLine: Missing GNU Shortcuts

[RolandMarchand]

Some GNU Bash/Readline shortcuts are not implemented in LibLine as of the latest commit c136280891.

Missing shortcuts (in order of personal usage):

• Alt+b: Back (left) one word
• Alt+f: Forward (right) one word
• Alt+d: Delete the Word after the cursor
• Alt+t: Swap current word with previous
• Alt+u: UPPER capitalize every character from the cursor to the end of the current word
• Alt+l: Lower the case of every character from the cursor to the end of the current word
• Alt+c: Capitalize the character under the cursor and move to the end of the word
• Сtrl+_: Undo
• Ctrl+xx: Toggle between the start of line and current cursor position
• Alt+r: ~~Cancel the changes and put back the line as it was in the history~~ (N/A, Shell's history is copied, not reference)

Note: I may have missed some, especially related to history, but these are the most commonly used and best documented ones.

Shell already implements many existing GNU Bash shortcuts, so I am proposing to extend the list.

I am not sure if the existing shortcuts are specifically implemented in LibLine, or in Shell, Terminal, or elsewhere. Since GNU Readline implements those shortcuts, I think LibLine should, too, if it was to have them.

Edit: this issue might be a good fit for the label "good first issue."