zig: add snippets

[ddogfoodd]

add snippets for:

• if/else statement
• if statement error handling
• while loop
• while loop with continue expression
• for loop
• for loop with index
• function
• error
• switch
• enum
• enum with type specification
• struct
• struct with defaults
• union
• tagged union
• page allocator
• fixed buffer allocator
• debug allocator