Fix tasks in tasks.json

[Wilfred]

#16839 refactored the representation of tasks inside the VS Code extension. However, this data type is exposed to users, who can define their own tasks in the same format in tasks.json or .code-workspace.

Revert the data type to have a command field rather than a program field, and document the different fields. This code is also a little complex, so split out a cargoToExecution to handle the Task to Execution conversion logic.

After this change, any tasks.json with a command field works again. For example, the following tasks.json works as expected:

{
	"version": "2.0.0",
	"tasks": [
		{
			"type": "cargo",
			"command": "build",
			"problemMatcher": [
			  "$rustc"
			],
			"group": "build",
			"label": "my example cargo build task"
		}
	]
}

Fixes #16943 #16949

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit a758e34 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit a758e34 with merge f3f14d4...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing f3f14d4 to master...