gum write: Add option to locate the cursor at the beggining of the innitial value text, instead of at the end.

[inkblobdev]

I've been using a lot gum write recently, the default behaviour is good most of the time, just put the caret at the end after the default value.

But for one specific use case, review a commit message, it makes more sense to have it at the top to be able to review line by line from the start of the message.

A bool flag defining where do you want your caret when invoking gum write could be added, with the default behaviour when not using the flag being the same as current behaviour.