Refactor and cleanup proc macro crate

[Sheldonfrith]

There are lots of new features to add, but when originally creating this library I did not have time to do a refactor or code cleanup of the macro crate (the other crates I did).

I would like to tackle this before we start adding too many new features and it creeps towards spaghetti code.

[Sheldonfrith]

Going to refactor to use a compiler pipeline type structure:
Something like this:

struct CompilerPipeline {
    phases: Vec<Box<dyn CompilerPhase>>,
}

trait CompilerPhase {
    fn execute(&self, input: CompilationUnit) -> Result<CompilationUnit, CompileError>;
}

struct CompilationUnit {
    ast: syn::ItemMod,
    metadata: CompilationMetadata,
}

struct CompilationMetadata {
    custom_types: Vec<CustomType>,
    binding_numbers: HashMap<String, u32>,
    // etc
}

// Example phases:
struct CustomTypeCollector;
struct WgslHelperTransformer; 
struct TypeConverter;
struct CodeGenerator;

Benefits:
Each phase has clear inputs/outputs via CompilationUnit
Phases can be tested in isolation
Flow is explicit and linear
Easier to add new phases