Add heuristics for OpenEdge ABL

[nickswalker]

Description

.p is a common file extension which is getting mapped to OpenEdge ABL in most cases (except when matching a pattern for GNUplot files). This has been raised several times ( #4216, #4074, #3835, #3619, #1277). This PR adds patterns to prevent this. I also added a pattern for the language under the .cls extension, as it was similar and there were already samples in the repo.

There were already two OpenEdge .p samples, but one was a single-line hello world. I have replaced this sample.

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • https://github.com/chriscamicas/vscode-abl/blob/a3e1fe7445fb87ee16271e1527b3da5ca99b573d/abl-src/run.p#L6
    • Sample license(s): MIT
  • I have included a change to the heuristics to distinguish my language from others using the same extension.