gum write: support overriding the default keymap (ctrl+j,enter)

[sharunkumar]

Is your feature request related to a problem? Please describe.
I would like an option to remap the default keymap for gum write (Ctrl+J, Enter) to better suit my workflow.

Describe the solution you'd like
Allow overriding the default keymap using command line input flags or env variables.

Describe alternatives you've considered
N/A

Additional context
N/A

[sharunkumar]

I can work on a PR for this, if this is good to go!

[caarlos0]

sounds good @sharunkumar! happy to help/review :)