Adjust bash aliases to work with WSL2
Adjust bash aliases to work with WSL2

- Detect WSL1 vs WSL2 based on capitalisation of [Mm]icrosoft in uname
  output (not ideal, but there doesn't seem to be an 'official' way to
  distinguish between the two from inside)
- Use .exe extension when referring to Windows application (p4)
- WSL2 uses an ext4 image rather than a windows directory, so
  WSL_ROOTFS_WIN doesn't make sense anymore
- Replace WSL cygpath wrapper with alias to wslpath since this now
  returns a network path like \\wsl.localhost\... to access WSL files
  under windows instead of the wsl$ magic it returned before

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