Enhancement: support access specifiers for function block declarations

[engineerjoe440]

Changes:

• Enhances grammar to qualify function blocks with more broad access specifiers, including:
  • ABSTRACT (pre-existing)
  • PUBLIC
  • PRIVATE
  • PROTECTED
  • INTERNAL
  • FINAL
• Adds tests to validate 61131 source directly, without the use of *.TcPOU files.

Closing Thoughts:

Please let me know if there are any questions or concerns with the changes! I have a few other enhancements I would like to contribute, and I want to make sure that my contributions are effective, and in-line with the direction you want to maintain.

Thank you so much!

[engineerjoe440]

Thank you so much for your input and suggestions! I'm about to add some additional tests to test_transform to get some more coverage there. I'll try to keep that to one commit more for ease of review.

Thank you again for your willingness to guide and share your work!

[engineerjoe440]

Thank you again for letting me muddle my way through this, and become familiar with your code-base. I really appreciate it! I think with this most recent set of commits, this PR should be ready for your review. :smile: