msedit: Add at v1.2.1

[Jaredy899]

Summary
A simple editor for simple needs.

This editor pays homage to the classic MS-DOS Editor, but with a modern interface and input controls similar to VS Code. The goal is to provide an accessible editor that even users largely unfamiliar with terminals can easily use.

Resolves: #6567

Test Plan

Tested on Budgie and KDE. Opens and saves.

Checklist

• Package was built and tested against unstable
• This change could gainfully be listed in the weekly sync notes once merged

[Jaredy899]

@EbonJaeger No rush but I believe this one is ready.

[EbonJaeger]

Does it not need Rust 1.90?

[Jaredy899]

I just needed to add RUSTC_BOOTSTRAP=1 for non-nightly builds.

[EbonJaeger]

Rust 1.90 is now in Unstable, can this now build without the workaround?

[Jaredy899]

Still no. This is the error it throws.

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> src/lib.rs:4:1
   |
 4 | / #![feature(
 5 | |     allocator_api,
 6 | |     breakpoint,
 7 | |     cold_path,
...  |
12 | |     maybe_uninit_uninit_array_transpose
13 | | )]
   | |__^
   |
   = help: the feature `let_chains` has been stable since `1.88.0` and no longer requires an attribute to enable

[Jaredy899]

More context from edit.

Build Instructions
Install Rust
Install the nightly toolchain: rustup install nightly
Alternatively, set the environment variable RUSTC_BOOTSTRAP=1

Clone the repository
For a release build, run:
Rust 1.90 or earlier: cargo build --config .cargo/release.toml --release
otherwise: cargo build --config .cargo/release-nightly.toml --release

[EbonJaeger]

LGTM, thanks!