Make theia examples more compliant with VS Code

[benoitf]

Description

As discussed at the last Theia dev meeting, today we have Theia examples that are including theia extensions which have their counterpart as VS Code extensions.

For example vscode-java extension [1] is actively developed by Red Hat but @theia/java is not so active [2]

Also it seems now that VS Code support is the most active development part inside theia and in lot of Pull Requests, we need to remove internal Theia extension and deploy VS Code extensions instead to reproduce/test the PR

So, it makes sense to me to have example which is what we might get if we were using VS Code. So, having build-in VS Code extensions

IMHO it will help people that want to use Theia for 'VS Code' support and make more dog-fooding on this support.

It could be new example or updating current examples. (At the meeting we were in favour of updating the current one)

@akosyakov also reported that for TypeScript support, we may have still troubles with this VS Code extension. We might link/file a new issue.

Based on this first feedback, probably the update of examples has to be incremental.

related to #6183 as well

[1] https://github.com/redhat-developer/vscode-java
[2] https://github.com/eclipse-theia/theia/commits/master/packages/java