Feature: Handle common directories variables

[idillon-sfl]

This handles the variables defined in https://git.yoctoproject.org/poky/plain/meta/conf/bitbake.conf

• They do not appear anymore as undefined when shellcheck is installed
• They are suggested as completion items in variable expansions or python datastore variables, but not in other places, such as variable assignment
  

[deribaucourt]

Instead of creating a new array of variable names, I think you should rather make available Ziwei's scanned variables for:

• hover value
• completion
  And the bitbake variables document hovers.