Fix single-quoted environment values retaining their quotes

[Ishubhammohole]

Summary

• Strip matching single quotes using the existing quoted-value path, just as for double quotes.
• Add public-API tests for empty values, preserved interior whitespace, hashes and trailing comments, embedded double quotes, literal backslashes, and unquoted apostrophes.

Testing

• mvn -B -Dtest=QuotedValuesTests -Djavadoc.dir=target/generated-javadoc test before the fix: 2 failures, 1 pass.
• mvn -B -ntp -Djavadoc.dir=target/generated-javadoc clean test jacoco:report package with Java 11: successful, all 30 tests passed.
• mvn -B -ntp -Djavadoc.dir=target/generated-javadoc test after the final edit: all 30 passed.
• git diff --check: passed.
• Supplemental mvn checkstyle:check with changed-file includes used the default Sun rules and failed; the unchanged parser also reports 44 violations. The configured Docker-based pre-commit hook could not run because the Docker daemon is unavailable.

The Javadoc directory override keeps build output under target instead of modifying tracked documentation. Prepared with Codex assistance.

Fixes #81