fix: misplaced shellcheck spans

[thatRichman]

Describe the problem or feature in addition to a link to the issues.

This PR addresses the bug described in #307, wherein shellcheck highlight spans were sometimes misplaced. This was due to incorrect counting of the amount of leading whitespace stripped by CommandSection::strip_whitespace. To avoid duplicating logic from the aforementioned method, the whitespace counting portion was refactored into a separate method, count_whitespace so that both strip_whitespace and sanitize_command can make use of it.

Before submitting this PR, please make sure:

• You have added a few sentences describing the PR here.
• You have added yourself or the appropriate individual as the assignee.
• You have added at least one relevant code reviewer to the PR.
• Your code builds clean without any errors or warnings.
• You have added tests (when appropriate).
• You have updated the README or other documentation to account for these
  changes (when appropriate).
• You have added an entry to the relevant CHANGELOG.md (see
  ["keep a changelog"] for more information).
• Your commit messages follow the [conventional commit] style.

[thatRichman]

This -2 was mis-attributed to newlines and was actually a coincidental artifact of the number of line continuations in my test cases. Doh.

[a-frantz]

LGTM! Thanks for this!

[peterhuene]

Looks good, thanks for fixing this!