Cross-platform env variable syntax in vhdl_ls.toml

[davidgussler]

I'm requesting that the syntax for environment variables be treated the same way for both Windows and Linux variables so that a vhdl_ls.toml can be checked into version control along with a project and work in any OS without modification.

This is mainly for handling vendor libraries that are installed on a machine, and not directly part of the project. See differences highlighted below:

Windows:

UNISIM.files = [
  '%XILINX_VIVADO%/data/vhdl/src/unisims/unisim_VCOMP.vhd',
]

Linux:

UNISIM.files = [
  '$XILINX_VIVADO/data/vhdl/src/unisims/unisim_VCOMP.vhd',
]

It would be ideal if Windows variables could be handled the same way as Linux variables. Thank you.