Add .editorconfig, to help contributors honor coding style.
Add .editorconfig, to help contributors honor coding style.

Setting the indentation for tcl and cc to 4 characters which
is mostly what is happening in this repository. There are
a few tabs here and there, but they will vanish with changes
happening there

The .editorconfig file is supported by a vast number of IDEs and
editors, so hopefully this will make it simpler for would-be
contributors.

Signed-off-by: Henner Zeller <259ad50fabbfd39355f36ecd6c0afca541ea66ef@acm.org>

antmicro/yosys-symbiflow-plugins,SymbiFlow/yosys-symbiflow-plugins,chipsalliance/yosys-f4pga-plugins,SymbiFlow/yosys-f4pga-plugins,SymbiFlow/yosys-f4pga-plugins,chipsalliance/yosys-f4pga-plugins,antmicro/yosys-symbiflow-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-f4pga-plugins,antmicro/yosys-symbiflow-plugins