CMake: Add install rules and rpath

[ADKaster]

If someone wants to run ladybird out of /usr/local or .local/bin (and take advantage of that sweet .local/Lagom default path I added) there should be some install rules added.

• check whether any modifications are needed to Lagom CMake lists to install from FetchContent'd repos.
• add install rules to ladybird, probably in a sep CMake file under CMake/ like Jakt has
• ensure the default rpath rules to make CMake not use build rpath for install are present.