feat: xmake | support plugin installation and packaging

[qudix]

• when you build (or run xmake install) it will search for the environment variable XSE_SF_MODS_PATH or XSE_SF_GAME_PATH and copy install files (by default ".dll" and ".pdb") to the root paths listed. More files can be added using add_installfiles.
  • the install path above can be overwritten using set_installdir
• when you run xmake package it will package the same files from above into a zip archive named after the target and the version.
• these events can be overwritten on a per-target basis if you wish to make your own install/package logic.