Merge branch '26-update-assembly-framework' into 'master'

Add framework for graphics-pipeline tests in spirv_assembly.

This allows single test code to be exercised in all shader kinds (except
for compute shader).

Implement assembly boilerplate as a StringTemplate with parameters in
certain sections that can be specialized.

Extend `StringTemplate` to accept optional parameter expansions, where a
missing parameter isn't an error.

Make `SpirVAsmSource::operator<<()` work on any parameter type.

See merge request !76