Used thiserror for the error enum
Moved the contents of the error module to the main module
Made the error an actual enum and not just a string
Splitted the write function on the feature manager into a function for
checking for feature availability

Splitted the backend into two more modules (features, errors)
Added a shorthand type for the backend Result
Reorganized function ordering
Moved from using a struct to write image dimension to a function
Started documenting the backend

Moved the global writing part to it's own function
Merged the texture and the global loop
Applied some suggestions from kvark
Added a topological sorting of the call graph before writing
Removed the struct block loop and now every struct is written
Organized the main backend file a bit

The backend now works in a similar way to the msl backend
Should require less loops, allocations and backtracking
Overall just better to work with it

Added a build script which reads a file with the glsl keywords and
generates a slice to be used in the Namer

Added a way to build a call graph

* [glsl-in] use TypeInner::scalar_kind for casts

* [glsl-in] Use .ok_or for casts