Switch to Myers diff

[SuperAuguste]

Fixes #979;

I originally ripped off gopls' Myers Difference Algorithm implementation, but realized that it did not fit our usecase, so I instead wrote about 2000 lines of code for a Zig DiffMatchPatch impl w/ Travis, which is what this PR uses! :)

Confirmed working by adding a new, correct diff patch implementation (which we should probably start using in the future) to diff.zig and getting tests running.

[SuperAuguste]

Extremely weird CI failure... (Compiler issue?)

[llogick]

Extremely weird CI failure... (Compiler issue?)

Weird.
0.11.0-dev.1570+693b12f8e

LLVM Emit Object... invalid type ref
!163020 = !DILocalVariable(name: "v_m", scope: !163021, file: !4047, line: 179, type: zig2: /home/workaccount/Work/llvm-project-15/llvm/include/llvm/Support/Casting.h:578: decltype(auto) llvm::cast(From *) [To = llvm::ValueAsMetadata, From = const llvm::Metadata]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

[SuperAuguste]

Sadly, while this drastically improves performance, this messes up the cursor position on format by moving it to the start of the line. Looking for a solution / better algorithm we could use :)

Solution found: implementing diffmatchpatch in its entirety, in Zig, live on stream :P

See the repo