feat(config): support detecting config from project manifest

[orhun]

Description

This PR updates the config detection mechanism to support the following cases:

cliff.toml	project manifest (e.g. Cargo.toml)	use config from:
✅	✅	cliff.toml
✅	❌	cliff.toml
❌	✅	Cargo.toml
❌	❌	builtin

Also, you don't need to specify the name of the manifest file anymore. So if you configured git-cliff from Cargo.toml, running git cliff is simply enough.

$ echo Cargo.toml | head

[workspace.metadata.git-cliff.changelog]
...
...

$ git cliff

# is same as
$ git cliff --config Cargo.toml

Motivation and Context

closes #568

How Has This Been Tested?

Locally.

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.
• I have formatted the code with rustfmt.
• I checked the lints with clippy.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.93103% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 41.07%. Comparing base (f26fbe0) to head (cd74567).

Files	Patch %	Lines
git-cliff-core/src/config.rs	40.75%	16 Missing ⚠️
git-cliff/src/lib.rs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #571      +/-   ##
==========================================
+ Coverage   41.07%   41.07%   +0.01%     
==========================================
  Files          15       15              
  Lines        1052     1069      +17     
==========================================
+ Hits          432      439       +7     
- Misses        620      630      +10     

Flag	Coverage Δ
unit-tests	41.07% <37.94%> (+0.01%)	⬆️

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[alerque]

I tested this on the project I was working on the other day and it did what I expected and found the config info I had in Cargo.toml without needing to manually add --config Cargo.toml on the command line.