bashrc: Add support for Windows Subsystem for Linux
bashrc: Add support for Windows Subsystem for Linux

- Detects when running in WSL as the kernel version indicates it is
  written by Microsoft
- Adds a shell function to get a Windows environment variable from WSL
- Exports the WSL windows root to an environment variable
- aliases cygpath to wslpath, and translates //wsl$ paths
- Auto mounts (links) Windows substituted drive letters

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