Make wasmworker-proc-macro a feature dependency

[paberr]

Summary

• Adds a macros feature to wasmworker that re-exports wasmworker-proc-macro
• Users can now depend on just wasmworker = { features = ["macros"] } instead of adding both crates
• Updates demo and test crates to use the new feature instead of a direct dependency

Non-breaking

This is a purely additive change — no existing behavior is affected. The wasmworker-proc-macro crate continues to work standalone.

Test plan

• cargo +nightly clippy --all-features
• cargo +nightly fmt --all -- --check
• cargo test --doc
• Integration tests pass (4/4: chromium + firefox, web + vite)

Closes #8

[Copilot]

Pull request overview

This PR makes the wasmworker-proc-macro crate available through the main wasmworker crate via a new macros feature, and updates internal demo/test crates to use the re-exported macros.

Changes:

• Added a macros feature to wasmworker and gated re-export of wasmworker-proc-macro behind it.
• Updated demo and test crates to import proc-macros from wasmworker instead of depending directly on wasmworker-proc-macro.
• Adjusted workspace dependency/lockfile entries to reflect the new dependency shape.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/lib.rs	Re-exports proc-macros behind cfg(feature = "macros").
Cargo.toml	Adds macros feature + optional dependency; updates workspace dependency features.
demo/src/lib.rs	Switches imports to wasmworker::webworker_fn.
demo/Cargo.toml	Removes direct wasmworker-proc-macro dependency.
test/src/raw.rs	Switches imports to wasmworker::webworker_fn.
test/src/convert.rs	Switches imports to wasmworker::webworker_fn.
test/src/channel.rs	Switches imports to wasmworker::webworker_channel_fn.
test/Cargo.toml	Removes direct wasmworker-proc-macro dependency.
Cargo.lock	Removes wasmworker-proc-macro from demo/test dependency lists.

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