[vscode] Extend TextEditorLineNumbersStyle with Interval

[rschnekenbu]

What it does

fixes #13447

The value Interval=3 was missing for the enum TextEditorLineNumberStyle. This PR adds this style, an extension can now set this style.

contributed on behalf of STMicroelectronics

How to test

• Install provided extension, that adds a command 'Set Line Style Number'. It displays a quickpick menu to choose the line number style for the active text editor.

• src: lineoptions-extension-0.0.1-src.zip
• zipped vsix: lineoptions-extension-0.0.1.zip

• On theia master, try to switch the line style options on a text editor, especially choosing Interval. Nothing shows up.
• After applying this PR, try again. Now setting the style to 'Interval' will display the line numbers every 10 lines + the current line number.

Follow-ups

none

Review checklist

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

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