#5843: add overtype edit mode

[noktoborus]

it is base implementation of overtype mode from https://github.com/helix-editor/helix/issues/5843

https://github.com/user-attachments/assets/b5cc4ccf-ca9f-470a-8f9f-544d03f81c7f

[noktoborus]

now helix fails if I switch to overtype mode and type one symbol in empty scratch buffer:

thread 'main' panicked at C:\<...>\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ropey-1.6.1\src\slice.rs:360:41:
called `Result::unwrap()` on an `Err` value: Char index out of bounds: char index 2, Rope/RopeSlice char length 1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[haxfn]

This feature is useful! Replace mode in vim rocks, one of the few times I have to jump from helix to vim to use Replace mode.

This merge request needs a review. Somebody please look at it.

[mattkang]

This seems like an essential feature.

[413x1nkp]

Sorry for bump, but what's the status on this? Was this pull request succeeded? I'd love to have this feature for ASCII art....

[noktoborus]

I've lost interest to helix. The implementation of this feature is not expensive, and someone could take the changes from this PR and update it to a fresh master, or write new code and fix the problem described in https://github.com/helix-editor/helix/pull/11265#issuecomment-2259284312 (perhaps it has already been solved).

[Axlefublr]

dear github, please add the sob emoji react for this exact usecase.