Ensure contributed problem patterns are found correctly

[martin-fleck-at]

What it does

• Resolve pattern names correctly by removing leading '$'
• Introduce utility function for dealing with variable names
• Ensure 'ready' promises are already available with object creation

Fixes #12725

How to test

• Follow instructions in bug report or checkout https://github.com/eclipse-theia/theia/tree/martin-fleck-at/issue-12725-test
• Run Electron App (either through launch config or command line)
• Create a task in a workspace with the following content:

{
   "version": "2.0.0",
   "tasks": [
      {
         "label": "echo",
         "type": "shell",
         "command": "echo somefile.abc:1:4: warning: the roof is on fire ",
         "problemMatcher" : ["$myproblemmatcher"]
      }
   ]
}

• Run Task 'echo' and check the Problems view:
  

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]

Thank you, looks good to me and works as expected.
I think we should add some docs to asVariableName and fromVariableName though, so that users know what happens when they pass variables/non-variables to each of the methods.

[jfaltermeier]

Should we add some tsdocs to these new util methods?

[martin-fleck-at]

Good point! I pushed an update with some comments.