add basic documentation to lualibs.lua
merged lualibs.lua back into dtx; adapt style to match luaotfload.dtx
add some documentation to ``lualibs-basic.lua`` and ``lualibs-merged.lua``
merge main failes into dtx

included are now:

* lualibs.lua
* lualibs-basic.lua
* lualibs-extended.lua
[doc] document partial loading
[doc] update list of extended set
[doc] describe the packaging
remove ``lualibs-basic.lua`` and ``lualibs-extended.lua`` from repository