Add PDDL

[celrm]

Description

Adds support for PDDL.

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&type=Code&ref=searchresults&q=extension%3Apddl+NOT+nothack
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/MylerRL661/AI-Pathfinding
      • https://github.com/EL2425/Drone_Project_2017-
    • Sample license(s):
      • https://github.com/MylerRL661/AI-Pathfinding/blob/main/LICENSE
      • https://github.com/EL2425/Drone_Project_2017-/blob/master/LICENSE.md
  • I have included a syntax highlighting grammar: https://github.com/jan-dolejsi/vscode-pddl

[lildude]

See inline comment.

We also don't need quite so many nearly identical samples, not the large ones. Please remove the samples that are collapsed in the diff (clear indicator they're too big) and pick the most diverse two or three of the remaining samples and remove the others.

[celrm]

See inline comment.

We also don't need quite so many nearly identical samples, not the large ones. Please remove the samples that are collapsed in the diff (clear indicator they're too big) and pick the most diverse two or three of the remaining samples and remove the others.

Thank you, I have applied the suggestions now. Please let me know if there is anything else.

[lildude]

Running script/list-grammars and committing the changes should fix the tests.