feat(cli): Dry run

[alexpovel]

This change adds a new flag --dry-run that when
given, prevents otherwise destructive file
overwrite operations and instead prints a rich
diff. The diff contains names of files which would be modified, and a git-like diff of what
modifications would be made.

The diff is human-readable, and not designed to be easily machine-readable.

The change leverages the similar crate, on which we transitively depended on already anyway.

Closes #152.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.78%. Comparing base (3beac24) to head (4932c6d).

Files with missing lines	Patch %	Lines
src/main.rs	94.11%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #156      +/-   ##
==========================================
+ Coverage   86.72%   86.78%   +0.06%     
==========================================
  Files          32       33       +1     
  Lines        1876     1900      +24     
==========================================
+ Hits         1627     1649      +22     
- Misses        249      251       +2     

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[alexpovel]

Looks like:

[alexpovel]

There's already a search mode, and as opposed to normal diffing tools, we know the exact actions we took. Thus it makes sense to leverage that search mode, and not do (much) extra work to see what changed. Results now look like:

which looks different from "normal" git-style diffs, as lines are never grouped. There's also no + and - prefix. I'd think this diff is for human consumption, where the colors and word-style diff make sense.