vi mode: use alt + key to execute normal mode

[tmuaz]

Description

Allow executing (and entering normal mode) normal mode commands by holding down alt in insert mode (or any other modes for that matter).

In regular vim this works as usual, however isn't handled by reedline.

Usecase

I commonly use this to go back to the previous command (with alt + j or alt + k), or go to the beginning with alt + I or delete the entire line and and go back to insert mode with alt + S

References

bash's readline and zsh's line editor both do this by default.