Improve line editor movement. Add "delete to end of word" command.

[svi-n]

What does this PR do?

• Adds "delete to end of word" command for line editing. Bound to alt-d.
• Adds additional keybinds for move forwards/backwards by word (alt-f and alt-b).
• Updates "move forward/backwards by word" commands to skip all intervening space between the cursor and the word e.g. in the line "A_ _ _B" with the cursor at B, moving backwards by word will place the cursor at A instead of moving one space back per key press.

Provide links to any related discussion on lines.

Discussion here.

How should this be manually tested?

• Type text in live or edit mode. Use alt-f, alt-b, alt-d keybinds.

Any background context you want to provide?

• Chosen new keybinds match existing use of emacs keybinds.

If the related Github issues aren't referenced in your commits, please link to them here.

I have,

• updated CHANGELOG.md
• updated the documentation
• updated help_mode.c (if applicable)
• run make format on each commit
• run tests

[scanner-darkly]

@svi-n - sorry, forgot to mention - could you also update help_mode.c?

[svi-n]

Ack - I'll send a change to update it.

[scanner-darkly]

@svi-n we're getting close to a release, do you think you'll be able to update help_mode.c?

[svi-n]

#267