Add shader I64 and U64 support

[atlv24]

Connections
Part of an incoming series of PRs needed by #5123
Others in this series: #5155
Addresses #4424
Ultimately for bevy meshlets pipeline bevyengine/bevy#10164

Description
Shaders currently do not support 64 bit integer types. Add support for these gated by a feature flag when the capability is present.

Testing
A snapshot test called i64.wgsl has been added to test parsing, type casting, and other basic operations.

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.

[cwfitzgerald]

Howdy! wgpu test fairy here: could you add execution tests for this in wgpu. We need at least one test, but there could be other things:

• struct layout should be tested in struct_layout.rs

Stale

[teoxoy]

LGTM

[teoxoy]

This needs a rebase or another merge of trunk into it.