Fix semantic tokens offset due to document updates

[0dinD]

Fixes #1597, together with eclipse-jdtls/eclipse.jdt.ls#1552

The fix is basically taken straight from the implementation of the TypeScript semantic token provider, see this line.

[testforstephen]

LGTM

[fbricon]

travis chokes on

ERROR: /home/travis/build/redhat-developer/vscode-java/src/semanticTokenProvider.ts[50, 13]: Identifier 'iv' is never reassigned; use 'const' instead of 'let'.

[0dinD]

Oh, sorry, I just copied the code from the TypeScript implementation. I'll fix it later today.

[0dinD]

@fbricon I've fixed up the code now.

[fbricon]

Thanks @0dinD, this seems to make a big difference, even without eclipse-jdtls/eclipse.jdt.ls#1552

[0dinD]

No problem!
I think the reason why this PR also mitigates the issue fixed in eclipse-jdtls/eclipse.jdt.ls#1552 is because the extension now waits until the document changes have settled before actually applying any semantic tokens from the server. But still, there could be an edge case where we do need eclipse-jdtls/eclipse.jdt.ls#1552, so as long as there is no problem with that PR I think it's a good idea to also apply that fix. Better safe than sorry, right?

When the document and compilation unit become out-of-sync on the server side without the client knowing, the token offset will become "stuck" until you change the document again, which is very annoying.