feat: Custom file extension for `$EDITOR` tmpfile

[fuguesoft]

Enhancement

Would it be possible to determine the file extension for the $EDITOR tmpfile?
I like to use .md for as many non-programming text files as I can and have my
lsp set up to work with them in a large capacity. I use snippets to quickly
enter codeblocks, lists, formatted hyperlinks, ...etc.

I was suggested to set an autocommand to set ft=markdown and that's doable but
I'd like to be able to control this option from concord's config.toml

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