* .github: add scheduled build

This will help PR authors determine if CI failures are caused by their
changes or by e.g. Rust releases.

* deps: disable proc-macro-error default features

proc-macro-error has an optional dependency on syn@1 whereas much of the
ecosystem has migrated to syn@2, resulting in dependents of test-case
having to compile syn twice. This avoids that by removing the optional
dependency.

* Don't blindly discard errors

This prevents syn::Error from propagating.

Updates #135.

* Use syn::ParseStream::error

This fails tests.

#135

* Use proc-macro2-diagnostics

Bump MSRV to 1.63 for the new yansi dependency (through
proc-macro2-diagnostics). See
https://github.com/SergioBenitez/yansi/blob/e8247a492e1b/Cargo.toml#L12.

…137)

* .github: bump actions/checkout to v4

* .github: do not fail fast

* .github: replace actions-rs with dtolnay/rust-toolchain

actions-rs is no longer maintained.

* .github: add rust cache action

This should reduce CI time.