Introduce `replace_leading_whitespace()` for #170

[mstackhouse]

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[codecov[bot]]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (5e5d244) 92.52% compared to head (4a94429) 92.24%. Report is 16 commits behind head on devel.

:exclamation: Current head 4a94429 differs from pull request most recent head 8314175. Consider uploading reports for the commit 8314175 to get more accurate results

Files	Patch %	Lines
R/replace_leading_whitespace.R	0.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #171 +/- ## ========================================== - Coverage 92.52% 92.24% -0.29% ========================================== Files 43 45 +2 Lines 2784 2799 +15 ========================================== + Hits 2576 2582 +6 - Misses 208 217 +9 ```

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