Auto merge of #16970 - Wilfred:fix_tasks, r=Veykril

Fix tasks in tasks.json

#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

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