Package ecosystems that know how to install CMake projects can now
package nanobind for distribution without using submodule'd
dependencies.

nanobind does not install any compiled library artifacts. Instead, all
necessary source files and supporting CMake files are installed to build
nanobind within another CMake project.

A CMake user can now adjust the CMake option `NB_USE_SUBMODULE_DEPS` to
choose whether nanobind will use and/or install the submodule'd external
dependencies. This is `ON` by default, allowing nanobind's CMake logic
to continue operating as it did previously.

Another CMake option was introduced, `NB_CREATE_INSTALL_RULES`, to allow
users to choose whether to enable or disable the installation rules.
Notably, if nanobind is added to a CMake project with
`add_subdirectory`, the installation rules are disabled, because
installing source files probably isn't the expected result of the parent
project(s). Install-rule generation still respects the standard CMake
`CMAKE_SKIP_INSTALL_RULES` variable.

Use CMake's `GNUInstallDirs` to provide initial variable values, then
create a variable `NB_INSTALL_DATADIR` to allow a user to specify where
nanobind's files are installed.