Fix non-Unix database path compilation

[daniel-vacic]

What changed

• replace the undeclared Error::InvalidPath in the non-Unix path conversion
  with the crate's existing RawPowerSyncError::ArgumentError
• add a Windows CI job that checks the powersync crate on windows-latest

Root cause

The Windows-only branch was not compiled by the Linux CI job, so the undeclared
type reached main. The implementation now mirrors the Unix branch's error
shape, and CI exercises the actual Windows configuration.

Fixes #20.

Validation

• rustup run 1.96.0 cargo fmt --all -- --check
• rustup run 1.96.0 cargo check -p powersync --target x86_64-pc-windows-msvc

The cross-target check now completes successfully from macOS; the added Actions
job will also compile on a native Windows runner.

[simolus3]

Thanks!