[cpp-electron] update app to use vscode-clangd for C/C++

[marcdumais-work]

Replacing @theia-cpp, that will soon be deprecated.

As well updated the scripts in package.json so that the vscode extensions are downloaded during build and used when starting the example application.

Signed-off-by: Marc Dumais marc.dumais@ericsson.com

[marcdumais-work]

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