vim: Add :w <filename>

[AidanV]

Closes #10920

Release Notes:

• vim: Adds support for :w[rite] <filename>

[ConradIrwin]

Thanks for this!

I don't think we should add this as an argument to SaveIntent, which is already pretty complicated, but instead use an internal vim action that calls save_as on the editor directly (and which errors if the file already exists).

I'd also really like to support filename completion here, but I don't think that needs to block.