Update Zig lexer and example

[davidpaquin]

It's been a while since the Zig lexer has been updated, and there have been a number of changes to the language since then. A rough summary of this patch:

• Adds new keywords that were not being tracked before, e.g. opaque and anytype
• Removes keywords that are no longer in the language, e.g. usingnamespace
• Replaces the test file with a larger one that covers more of the language
• Follows more best-practice lexer structuring/optimizations, e.g. combining common token sequences when possible