allow usage of filenamePart template in custom labels

[bsShoham]

This pull requests add to the custom label service the option to accept a new template variable in the patterns, which allows to insert into the label each of the parts of the full file name (including the extension) separated by .

usage example:

settings.json:

    "workbench.editor.customLabels.enabled": true,
    "workbench.editor.customLabels.patterns": {
        "**/unit-tests/**/*.spec.*": "Test ${filenamePart(1)}",
        "**/e2e/**/*.e2e.spec.*": "Test ${filenamePart(2)} ${filenamePart(1)}",
    }

The results would be:
WORKSPACE/units-tests/users.spec.ts -> Test users
WORKSPACE/e2e/users.e2e.spec.ts -> Test e2e users

closes #210166

also fixes some small spelling mistakes.

[bsShoham]

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[benibenj]

Thank you for creating a PR for this. I think it makes sense to tackle this case.
I see you want to introduce a variable filenamePart. I wonder if this might be a bit confusing.

I would propose the following:

• filename: name of the file without any extension
• extname: the entire extension
• extname(N): a specific part of the extension starting from the back

Example path: src/main.ex1.ex2.ex3

• filename: main
• extname: ex1.ex2.ex3
• extname(0): ex3
• extname(1): ex2
• extname(2): ex1
• extname(3): This does not work and would show the template (${extname(3)}) to indicate to the user that his pattern/template is invalid

[bsShoham]

@benibenj Thanks for the reply and the suggestions!
I've gone and made changes according to your proposal, would love it if you can have a look.

[benibenj]

Thanks for making the changes!
We are currently preparing for the release next week (this means no more changes can go in) so I'll review this in June and we can make it part of the June release