Interpolation support for wgsl, glsl, glsl-new, spirv (front and back), and glsl-out (#140)

* Add interpolation support to naga core and wgsl

* Add glsl-new fragment shader support to examples/convert.rs

* Add interpolation support to glsl_new

* Add interpolation support to spirv frontend

* Add interpolation support to spirv backend

* Add interpolation support to glsl backend

* Add interpolation support to old glsl backend

* Fix interpolation field syntax in glsl backend

* Formatting fix

* thanks clippy

* Add wgsl struct interpolation decoration

* Change wgsl interpolation syntax

* Modify interpolation enum

* Changes responding to review

* Remove interpolation struct member qualifier

* code changes after review