fix: ensure --check validates copyright holder with -c

[Aswinr24]

Description

This PR ensures that when using --check with -c "Copyright Holder", addlicense verifies that the copyright holder matches the expected value instead of just checking for a license header.

Additionally, this PR:

• Adds support for .spec files, ensuring they receive license headers.
• Fixes an issue where addlicense incorrectly flagged empty files as missing a header. Empty files are now ignored when checking or adding headers.

Changes Made

• Added extractCopyrightHolder to correctly extract and validate the copyright holder using regex.
• Ensured default behavior remains unchanged when -c is not provided.
• Added support for .spec files.
• Updated logic to skip empty files during license checks and additions.

Test Output:

addlicense -c "Google LLC" --check testdata/initial/file.cs

Before Fix:

testdata/expected/file.cs

After Fix:

testdata/expected/file.cs: incorrect copyright holder (expected: Google LLC, found: Meta Platforms)

Tests & Verification

All unit tests passed successfully.

Closes #164 , Closes #162 , Closes #163