qmlls: pass build-dirs easily

Currently, using qmlls requires passing the build folders by hand, which is not super user-friendly.

Add a CMake option to automatically generate .qmlls.ini files, such that users can activate the option and use qmlls without having to bother about the current build folder. This option requires generating the .qmlls.ini files in the source directories but it does not require a custom implementation for each Editor (like clangd and compile_commands.json, which has a different custom implementation for each editor).