scm: add support for strikeThrough

[vince-fugnitto]

What it does

The pull-request adds support for strikethrough decorations for the scm-view for both our internal @theia/git extension and source-control-provider plugins. The changes include modifying our internal @theia/git extension to mark DELETED resources with a strikethough (line-through).

How to test

@theia/git:

1. start the application with a workspace under version control
2. perform changes in the workspace including adding new files, modifying existing files and deleting files
3. confirm that the deleted files are decorated with a strikethrough

vscode.git builtin:

1. remove @theia/git from the example application(s)
2. include the custom vscode.git and vscode.git-base builtins git-builtins.zip (these builtins are modified to include the strikethrough on all resources)
3. start the application with a workspace under version control
4. perform changes in the workspace including adding new files, modifying existing files and deleting files
5. confirm that all resources are decorated with a strikethrough

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

[msujew]

Looks good to me 👍

[vince-fugnitto]

I'll merge this one after 1.33.0.