Move "internal" crates into the `internal` directory. This first batch
includes most of sixtyfps_runtime but leaves the rendering backends
alone for now.
pre-commit applied some cleanups to the moved files:
- Consistent newline at end of file policy
- trimming trailing whitespace
- Formatting Cargo.toml files.
- build corelib_macros between common and the compilerlib so that the
crates.io cache has time to propagate
- add a feature to the default backend otherwise it doesn't compile
On Windows 10, the creation of symlinks by normal users requires
enabling the developer mode, which may or may not be acceptable in
corporate environments with restricted IT setups.
We introduced the symlinks for the shared special key codes mapping,
which instead this patch places into a shared sixtyfps-common crate.
