Rename old APaRSer code

[CBielstein]

The APRS parser was originally written as a standalone library and titled APaRSer, as a play on APRS and the word parser. Let's rename it to something professional.

For this issue:

• [ ] Rename APaRSer project to AprsParser or something similar
• [ ] Move namespace of various classes (APRS, AX.25, etc. classes) to AprsSharp.Parsers or something equally appropriate
• [ ] Update references in tests
• [ ] Update project name, class names, and namespaces in tests to reflect new changes in the source code

[CBielstein]

It's probably easier to do this one after #6 so that dotnet build and dotnet test can be used to validate.