Allow task types to be "disabled"

[alexr00]

There are some cases where a task type is not useful. In this case, the task type shouldn't be surfaced anywhere. One way to accomplish this would be to add a when property to task definitions. We can also add three built in contexts: ShellExecutionTasksSupported, ProcessExecutionTasksSupported, and CustomExecutionTasksSupported.

[alexr00]

Example of the current contribution point:

    "taskDefinitions": [
      {
        "type": "typescript",
        "required": [
          "tsconfig"
        ],
        "properties": {
          "tsconfig": {
            "type": "string",
            "description": "%taskDefinition.tsconfig.description%"
          },
          "option": {
            "type": "string"
          }
        }
      }
    ],

Proposal is simply to add a when:

    "taskDefinitions": [
      {
        "type": "typescript",
        "required": [
          "tsconfig"
        ],
        "properties": {
          "tsconfig": {
            "type": "string",
            "description": "%taskDefinition.tsconfig.description%"
          },
          "option": {
            "type": "string"
          }
        },
        "when": "ShellExecutionTasksSupported"
      }
    ],

This would provide further benefits too. The typescript task provider could add a context for when there's a tsconfig file, and include that in the when.

[alexr00]

This is now finalized.