Add an .editorconfig file to all SymbiFlow repositories

[`.editorconfig` is a file](https://editorconfig.org/) which tells many editors how to set up their auto formatting configurations. See this example in the Yosys repository -> https://github.com/YosysHQ/yosys/blob/master/.editorconfig

We should create an `.editorconfig` for SymbiFlow projects and then add it to all our repositories.

We probably need a configurations for;
- [ ] Python files
- [ ] C++ files
- [ ] TCL scripts
- [ ] Bash scripts
- [ ] Makefiles
- [ ] cmake files
- [ ] Verilog files
- [ ] Exclude `third_party` directory

For Python files, it should match what `.yapf` settings we are using.
For C++ files it should match the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).
For Verilog files it should match the [LowRISC Verilog Style Guide](https://github.com/lowRISC/style-guides/blob/master/VerilogCodingStyle.md).

What else have I missed?