The F1 service should produce `#if` instead of `if_CSharpKeyword` when
the cursor is on the `#if` symbol.

Add other tasks for the other preprocessor symbols related to
conditional compilation. The `#if` and `#else` fail, because the model
finds child nodes for `if` and `else`, respectively. The tests for
`#elif` and `#endif` pass, because the model does not find child nodes
for those tokens.

Because the `#if` and `#else` preprocessor tokens contain the text for keywords (`if` and `else`), the `TryGetTextForKeyword` would lock to those keywords before the `TryGetTextForPreProcessor` looked for the preprocessor token. By switching the order of those two tests, the longer preprocessor tokens are found first.

The "if" and "else" nodes may appear as "if" / "else" or "#if" / "#else".

If the parent isn't the if statement or else clause, don't treat them as keywords. Then, the later processing will find them as preprocessor tokens and correctly route the request.