Embed latest Java textmate grammar

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With Java 15 support coming, new keywords sealed/non-sealed/permits need a textmate grammar update. This has been mostly done upstream in https://github.com/atom/language-java/blob/master/grammars/java.cson (although non-sealed was skipped)
Since vscode hasn't been updated yet, nor Theia nor Che-Theia, it seems safer for vscode-java to embed the latest textmate grammar for a while, until we can rely on those editors to do the right thing.