vscode: add support for contributes.icons contribution point

[tsmaeder]

Bug Description:

Some of the editor toolbar icons are now visible when installing gitlens

Steps to Reproduce:

1. Install gitlens
2. Open a typescript editor
3. Observe: on the right side of the editor tab-bar, there is a toolbar, but some of the icons from the gitlens extension are invisible, even though room is allocated for the icons

Additional Information

• Operating System: WIndows
• Theia Version: master

[vince-fugnitto]

@tsmaeder I don't believe we support the icons contribution point yet which gitlens uses to contribute it's icons:

• https://github.com/microsoft/vscode/blob/main/src/vs/workbench/services/themes/common/iconExtensionPoint.ts
• https://github.com/gitkraken/vscode-gitlens/blob/ad8f85639e8879fa6e173002c13343051953c4c4/package.json#L7237-L7244