feat: enable underscore integer separation for decimal values (ints)

[marcules]

This fixes #202

Checklist

• Add test cases to all the changes you introduce
• Update the documentation for the changes

[coveralls]

Pull Request Test Coverage Report for Build 3818872307

• 9 of 11 (81.82%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 99.297%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/yamlfix/adapters.py	8	10	80.0%

Totals
Change from base Build 3787161522:	-0.5%
Covered Lines:	424
Relevant Lines:	427

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💛 - Coveralls

[lyz-code]

Thanks again for your time. I'd simply add another test to be sure

[lyz-code]

Just to make sure I'd add a test where the source has underscores, the config is set to undersscore_integer = False and make sure that the reverse procedure works well too

        fixed_source = dedent(
            """\
            small_ints: [1, 10, 100]
            big_ints: [1000, 10000, 1000000, 100000000000000000000]
            """
        )
        source = dedent(
            """\
            ---
            small_ints: [1, 10, 100]
            big_ints: [1_000, 10_000, 1_000_000, 100_000_000_000_000_000_000]
            """  # noqa: E501
        )

        config = YamlfixConfig()
        result = fix_code(source, config)
        assert result == fixed_source