## [why]
For 'windows' platforms, directly spawning a process (eg, called PATHNAME) bypasses the
usual windows shell machinery for determining which process to execute. Specifically,
the extensions in PATHEXT will not be used to determine the final executable. So,
`PATHNAME.bat`, `PATHNAME.cmd`, ... will *not* be executed even if on they exist on the
PATH; and this is counter to the usual expectation of a Windows user. Additionally,
built-in commands, such as `echo` and `dir`, will never be accessible as they do not
have a PATH to execute and, so, will never be found.
To use the usual machinery, giving access to PATHNAME.bat and `echo`, execute the PATHNAME
using the windows shell, eg `cmd /d/c PATHNAME`. Note this may expose the constructed
command line to the windows shell quoting vagaries (sadly, that may be part of the price).
Following Windows standards, the ComSpec environment variable is used to determine which
shell to use, with a fallback to the "modern", built-in `cmd` shell.
This changes the base16 theme back from #RRGGBB0f to #RRGGBB00,
reverting part of #934. That PR used the 0f encoding to produce ANSI
escape sequences 30-37 and 40-47 rather than 38;5 and 48;5 which require
256-color support. Unfortunately, it resulted in base16 using the wrong
colors becuase ansi_term does not support the bright variants (90-97 and
100-107) so it simply mapped them to the non-bright colors.
This PR makes combines the 00 and 0f alpha encodings into 00, and makes
them use the Color enum for the first 8 colors and Fixed otherwise. This
means the ansi-light and ansi-dark themes will work on terminals without
256-color support, and base16 will render bright colors correctly.
