refactor: executor functional options

[pd93]

This PR migrates the Executor to use the functional options pattern. This follows other similar changes to taskfile.Reader and taskfile.Snippet. This allows package API users to easily construct an Executor with sensible defaults and only supply a list of things they want to override.

Our internal CLI implementation has a special functional option in internal/flags called WithFlags which will gather all the CLI flags and pass them in as functional options.

[vmaerten]

Good refactor as always!

[andreynering]

Great work!

[andreynering]

In theory we could have an additional function that accepts all three, for shortness. Optional suggestion, though.

func WithIO(stdin io.Reader, stdout, stderr io.Writer) ExecutorOption {
	return func(e *Executor) {
		e.Stdin = stdin
		e.Stdout = stdout
		e.Stderr = stderr
	}
}

[pd93]

To me, it makes more sense to keep these separate if you're passing different values into each of them. Sometimes you might only want to pass in 1 or 2 of them and leave the others blank. However, I think if you're setting them all to the same value, we could have something like this:

func WithIO(rw io.ReadWriter) ExecutorOption {
	return func(e *Executor) {
		e.Stdin = rw
		e.Stdout = rw
		e.Stderr = rw
	}
}

This would be super handy in the new tests I've been writing where we want to write everything to a single buffer.

[pd93]

Couple of minor changes:

• Added the missing ExecutorWithVersionCheck functional option in cmd/task
• Added prefixes to the functional options like in taskfile.Snippet
  • This helps with any potential naming conflicts in the future.
• A bunch of docstrings for everything.