Add python implementation for wgsl-gen (#28355)
### Description
Replaces the npm-based `@fs-eire/wgsl-template` Node.js engine used by
the WebGPU EP's static build path with an in-tree, stdlib-only Python
implementation under `wgsl_templates`. The dynamic generator path is
removed from the build surface - the code logic remains as unreachable
and will be removed in a follow-up.
### Motivation and Context
The WebGPU EP's WGSL template-to-C++ codegen is currently driven by the
@fs-eire/wgsl-template npm package, so any build that touches a
.wgsl.template file needs Node.js installed and has to pull the package
over the network. This npm package is also no longer maintained.
