Invalid codepath for cpp_map(UPDATE)

[AutonomicPerfectionist]

Describe the bug
Calling cpp_map(UPDATE) results in a call to cpp_map_update(), which is not a function. The correct function appears to be cpp_map_merge(), in which case the top-level subcommand should be renamed to match.

See:

CMakePPLang/cmake/cmakepp_lang/map/map.cmake

Lines 82 to 83 in ea8a021

	elseif("${_m_lc_mode}" STREQUAL "update")
	cpp_map_update("${_m_this}" "${ARGV2}")

Introduced by #22. Tests only test the discrete cpp_map_merge() function and do not test that the top-level command delegates to it correctly.