feat: decoding improvements

[pd93]

These changes came from investigating #2065. This PR does not fix that issue (I will do that in a follow up PR), but it did uncover a few problems which I've addressed here:

• Refactored how snippets work
  • Moved from errors package to taskfile package (now part of the package API)
  • Functional options
  • Support for out-of-bounds lines/columns
  • Ability to hide the line/column indicators
  • A bunch of tests
• Added a debug function for TaskfileDecodeError. This was just helpful to track down the next issue
• Fixed the ast.Cmd Unmarshaler so that errors are correctly propagated to the user.
  • This is what is causing the unhelpful error in Allow to use vars in matrix #2065.
  • It should now return something like: err: cannot unmarshal !!map into []interface {} which much more accurately describes the problem.
• Fixed the Taskfile schema for defer statements when used in the shorthand commands array syntax

[vmaerten]

Way cleaner
I like the fact you splitted defer into another file!