This PR introduces two type alias to avoid the confusing const generic `pub struct Codegen<'a, const MINIFY: bool>`
* CodeGenerator - Code generator without whitespace removal.
* WhitespaceRemover - Code generator with whitespace removal.
Usage is changed to a builder pattern:
```rust
CodeGenerator::new()
.enable_comment(...)
.enable_sourcemap(...)
.build(&program);
```
