Fix #11. Improve preprocessor parsing

[AbigailBuccaneer]

Now preprocessor directives are lexed properly. The parser just adds all tokens after a preprocessor directive until the next newline to the PSI element.

Currently parsed and highlighted correctly:

#version \
300 \
es

#if !defined(__GLSL__)
#error Uh-oh!
#endif

void f() {
    int x = 3
    #ifdef FOO
    + 4
    #else
    * 5
    #endif
    ;
}

Not yet implemented:

• Associating #if/#ifdef/#ifndef with the matching #else/#end. They're currently just parsed as two independent directives.
• Error highlighting when a directive is not the first non-whitespace token on a line. I tried to do it in the parser, but it doesn't play nice with other whitespace, so I think a better approach will probably be an error annotator. (Interestingly, CLion doesn't highlight errors about this for C/C++.)
• Handling the contents of the directives. We would probably want:
  • #version and #extension to set properties on the GLSLFile, to be used for error annotation and autocompletion
  • #define to actually define tokens, which are then inserted into the token stream when coming across them in advanceLexer
  • #if/#ifdef/#ifndef to be evaluated properly (this would require some constant expression analysis).

[Darkyenus]

Looks good