[naga wgsl-in] Proper singular generic in vec and matrix

[sagudev]

Connections
Fixes #4473

Description
next_scalar_generic was replaced with new singular_generic, that only parses into a Type, that is checked for scalar later when we convert it to naga-ir. Probably all next_scalar_generic(_with_span) would needed to be converted similarly to support type aliases.

Testing
Existing test were updated to also test type aliases in matrix/vec generic.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[sagudev]

Servo's CTS results:

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,expression,access,vector:concrete:*

    PASS [expected FAIL] subtest: :vector_decl="const";vector_width=4;element_type="i32"
    PASS [expected FAIL] subtest: :vector_decl="const";vector_width=4;element_type="u32"
    PASS [expected FAIL] subtest: :vector_decl="const";vector_width=4;element_type="f32"
    PASS [expected FAIL] subtest: :vector_decl="const";vector_width=4;element_type="bool"
    PASS [expected FAIL] subtest: :vector_decl="let";vector_width=4;element_type="i32"
    PASS [expected FAIL] subtest: :vector_decl="let";vector_width=4;element_type="u32"
    PASS [expected FAIL] subtest: :vector_decl="let";vector_width=4;element_type="f32"
    PASS [expected FAIL] subtest: :vector_decl="let";vector_width=4;element_type="bool"
    PASS [expected FAIL] subtest: :vector_decl="var";vector_width=4;element_type="i32"
    PASS [expected FAIL] subtest: :vector_decl="var";vector_width=4;element_type="u32"
    And 6 more unexpected results...

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,types,alias:match_non_alias:*

    PASS [expected FAIL] subtest: :case="template_param"
    PASS [expected FAIL] subtest: :case="predeclared_alias"

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,types,alias:no_indirect_recursion_via_matrix_element:*

    PASS [expected FAIL] subtest: :target="f32"

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,types,alias:no_indirect_recursion_via_vector_element:*

    PASS [expected FAIL] subtest: :target="i32"

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,types,matrix:valid:*

    PASS [expected FAIL] subtest: :case="alias"

OK /_webgpu/webgpu/cts.https.html?q=webgpu:shader,validation,types,vector:vector:*

    PASS [expected FAIL] subtest: :case="aliased_el_ty"

[teoxoy]

Good stuff! Thanks!