Despite their being an actual `egui::Margin` struct, Textedit has a
`margin()` builder function that supports only `Vec2` types and thereby
only symmetric margins. This PR changes the function to accept
`egui::Margin` type instead making it more congruent with overall egui
logic as well as supporting asymmetric margins.
P.S: I tried to run all checks but I had to modify `./rust-toolchain` to
1.67.0 to get the checks to run on macOS.
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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>
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