Add script for Symbolator diagrams generation

[glatosinski]

Fixes #244

This PR adds a script generate_symbolator_plots.py that generates the symbol.svg files for all symbol.v files found in libraries, unless particular version or library is specified.

[glatosinski]

Currently, in the libraries, there are symbol.v files listed in the
List of modules with no inputs or outputs
for which the corresponding symbol.svg files are just empty diagrams.

Those symbol.v files have no inputs or outputs, e.g. https://foss-eda-tools.googlesource.com/skywater-pdk/libs/sky130_fd_sc_ms.git/+/refs/heads/master/cells/tapvgnd2/sky130_fd_sc_ms__tapvgnd2.symbol.v

Are symbol.v modules with not I/O and corresponding empty diagrams the expected behaviour and the empty diagrams should be generated for those symbol.v files?