Support static/shared builds and an ALIAS for the default

[misery]

It is currently not possible to build STATIC and SHARED libraries at the same time with CMake.

CMake Error at CMakeLists.txt:106 (add_library):
  add_library cannot create ALIAS target "llhttp::llhttp" because another
  target with the same name already exists.

You have add_library(llhttp::llhttp ALIAS llhttp_shared) twice in CMakeLists.txt.

Also it would be nice if the IMPORTED target in CMake could be llhttp:llhttp as an alias in addition to existing llhttp::llhttp_shared and llhttp::llhttp_static.
Currently we use this to detect it.

find_package(llhttp REQUIRED)

if(TARGET llhttp::llhttp_shared)
	add_library(llhttp::llhttp ALIAS llhttp::llhttp_shared)
elseif(TARGET llhttp::llhttp_static)
	add_library(llhttp::llhttp ALIAS llhttp::llhttp_static)
else()
	message(FATAL_ERROR "TARGET of llhttp not found")
endif()

add_library(OurLibrary)
...
target_link_libraries(OurLibrary llhttp::llhttp)