Fix 'pickString' evaluation

[sdirix]

Fixes the 'pickString' evaluation in tasks and launch configurations.

Fixes eclipse-theia#11585

Contributed on behalf of STMicroelectronics

What it does

Similar to 'promptString', 'pickString' can be used to replace variables in task and launch configurations. However 'pickString' values were not correctly handled which prevented them from being evaluated. This is now fixed.

How to test

Run the task from the following task.json

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "echo select foo or bar",
            "type": "shell",
            "command": "echo ${input:select-foo-or-bar}",
            "problemMatcher": []
        }
    ],
    "inputs": [
        {
            "id": "select-foo-or-bar",
            "type": "pickString",
            "description": "Select foo or bar",
            "default": "foo",
            "options": [
                "foo",
                "bar"
            ],
        }
    ]
}

Review checklist

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

Reminder for reviewers

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[AlexandraBuzila]

Works great and looks good to me!

[sdirix]

Thanks for the review! Upstream PR is now open: eclipse-theia#12100