[request] library support for filesets

[WRoenninger]

To revive the library discussion started in #55.

For a lot of the VHDL designs I've seen recently, there was heavy usage of different libraries for each of the sub IP designs.
As far as I am aware, the scripts generated by Bender all use the default library work as the generated scripts do not specify it.
Patching the generated scripts is a solution, but can be a bit of a hassle when writing the Bender.yml description for more complex VHDL sub-design with different used libraries.

I think there is the opportunity here to extend the functionality of the Bender.yml description for filesets by adding the field library or something similar:

 # Grouped source files may have additional include dirs, defines, target and library:
- include_dirs:
  - src/include
  - src/stuff/include
  defines:
  # Define without a value.
    EXCLUDE_MAGIC: ~
  # Define with a value.
    PREFIX_NAME: stuff
  target: all(asic, synthesis, freepdk45)
  library: core_lib
  files:
  - src/core/pkg.sv
  - src/core/alu.sv
  - src/core/top.sv

My Rust expertise is not the best though, but I think this could be quite easily added.