Detect authors in multi-line contributor lists

[Vaibhav200303]

Fixes #5138

Summary

ScanCode currently combines contributors from certain multi-line Markdown
contributor lists into a single author detection. In the reported govalidator
README, the final contributor is also omitted.

This change uses the source line information already retained by lexer tokens
to emit one AuthorDetection per contributor line.

The behavior is intentionally limited to explicit author/contributor lists
where:

• at least two entries follow the marker;
• each entry contains at least two name tokens;
• each entry contains a URL or email; and
• no unexpected token categories are present.

The split is applied only to the relevant parsed AUTHOR tree, avoiding
changes to the general copyright grammar.

Changes

• Detect individual authors in explicit multi-line contributor lists.
• Add a regression fixture based on the reported govalidator case.
• Update CHANGELOG.rst.
• Add the contributor to AUTHORS.rst.

Testing

• Regression fixture: 1 passed
• Copyright detector suite: 4738 passed, 13 xfailed
• End-to-end CLI scan: all 9 contributors detected separately
• git diff --check: passed

The repository-wide standard suite was also run before the change on Windows.
It contains pre-existing environment/platform-related failures unrelated to
this patch.