Feature idea: add a --file-extension option to gum write

[maxjacobson]

Hello!

I have a script which uses gum write to ask the user write some code. If the user types ctrl-e to open an editor, a tempfile is created with the .md extension:

gum/write/write.go

Line 162 in 0e98776

f, err := os.CreateTemp("", "gum.*.md")

Let's say I'm asking the user to write some go code. It would be nice if the editor had syntax highlighting for go. But because the extension indicates it's a markdown file, they have syntax highlighting for markdown.

Proposal: if I know I am asking a user to write go code, it would be nice to be able to specify the file extension:

go write --file-extension="go"

When I specify that, and the user presses ctrl-e, then gum should create a tempfile with the "go" file extension, and then the user will have a more pleasant time typing in their go code.

When the option is omitted, it would continue to create a markdown tempfile by default.

Thanks for considering!