feat: Support @ prefixes

[ianlewis]

Description:

Adds support for @ prefix on TODOs ala the old JavaDoc style.

e.g.

/**
 * @todo: foo
 */

Related Issues:

Fixes #516

Checklist:

• Review the CONTRIBUTING.md documentation.
• Add a reference to a related issue in the repository.
• Add a description of the changes proposed in the pull request.
• Add unit tests if applicable.
• Add a note in the CHANGELOG.md if applicable.
• Sign the Google CLA.

[codecov]

Codecov Report

Merging #759 (a323f4a) into main (ce98583) will increase coverage by 0.48%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #759      +/-   ##
==========================================
+ Coverage   80.50%   80.99%   +0.48%     
==========================================
  Files          19       18       -1     
  Lines        1144     1126      -18     
  Branches       14       14              
==========================================
- Hits          921      912       -9     
+ Misses        160      154       -6     
+ Partials       63       60       -3     

Files Changed	Coverage Δ
internal/todos/todos.go	99.16% <100.00%> (ø)

... and 1 file with indirect coverage changes