feat(args): add default value for --prepend option

[CatBraaain]

Description

git cliff # prepend to stdout
git cliff -p # prepend to CHANGELOG.md
git cliff -p OTHER_FILE.md # prepend to another file

If -p is given and path is omitted, prepend to CHANGELOG.md, like what --output option is.

related PR: #354

Motivation and Context

Align the behavior of --prepend with the existing --output option.
Since CHANGELOG.md is the target file in most cases, this makes the command more convenient to use.

How Has This Been Tested?

# cli test
cargo +stable-x86_64-pc-windows-msvc build --release

target/release/git-cliff -u              # printed unreleased
target/release/git-cliff -u -p           # added unreleased to CHANGELOG.md
target/release/git-cliff -u -p README.md # added unreleased to README.md

# website test
cd website
vp install
vp run build
vp run serve # The website builds successfully and the updated documentation is displayed correctly.

Screenshots / Logs (if applicable)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly (if applicable).
• I have formatted the code with rustfmt.
  • cargo +nightly fmt --all
• I checked the lints with clippy.
  • cargo clippy --tests --verbose -- -D warnings
• I have added tests to cover my changes.
• All new and existing tests passed.
  • cargo test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.61%. Comparing base (772e01d) to head (f4b9f8a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1549      +/-   ##
==========================================
- Coverage   48.65%   48.61%   -0.04%     
==========================================
  Files          26       26              
  Lines        2288     2288              
==========================================
- Hits         1113     1112       -1     
- Misses       1175     1176       +1     

Flag	Coverage Δ
unit-tests	48.61% <ø> (-0.04%)	⬇️

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