Ensure we keep task type and properly support custom execution

[martin-fleck-at]

What it does

• Make TaskProviderAdapter more resilient against undefined
• Ensure task type is not lost during conversion (used for custom exec)
• Ensure task type is considered during comparison

Fixes #12721

How to test

• Checkout https://github.com/eclipse-theia/theia/tree/martin-fleck-at/12721-test which adds some example tasks to Theia
• Launch the application using launch configuration 'Launch Electron Backend' (as it sets the plugin directory correctly)
• F1 > Hello from plugin-a (activates plugin)
• F1 > Run Task... > mytasktype
• Execute the shell, process and custom task to see if they are working as expected

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[jfaltermeier]

Thanks, lgtm!
I've tested with the TaskProvider from your branch.

[martin-fleck-at]

@vince-fugnitto @msujew @tsmaeder Does anybody want to have another look at this before we merge it?

[tsmaeder]

If it's good enought for @jfaltermeier, it's good enough for me.