perf(fmt): speed up file diffing (#30644)

This swaps out dissimilar for imara which is substantially faster at
diffing strings.

Note that this is a proof of concept and I did not have enough time to
make the output pretty. I just shows that the diff is fast. Applying
colors should be doable without changing much about the perf. I'm
willing to fix this up if I get an OK about the general direction.

Fixes #30634

---------

Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

Cargo.lock

@@ -362,6 +362,7 @@ dprint-plugin-markdown = "=0.20.0"
 dprint-plugin-typescript = "=0.95.15"
 env_logger = "=0.11.6"
 fancy-regex = "=0.14.0"
+imara-diff = "=0.2.0"
 libsui = "0.12.6"
 malva = "=0.12.1"
 markup_fmt = "=0.22.0"

Cargo.toml

@@ -49,9 +49,9 @@ deno_permissions = { workspace = true, optional = true }
 deno_semver.workspace = true
 deno_terminal.workspace = true
 deno_unsync.workspace = true
-dissimilar.workspace = true
 futures.workspace = true
 http = { workspace = true, optional = true }
+imara-diff.workspace = true
 import_map.workspace = true
 indexmap.workspace = true
 jsonc-parser.workspace = true