Fix #4726: On Windows, inform or warn about 'programs' path with space

[mpilgrem]

In module Stack.Config, function configFromConfigMonoid, a stack user is informed if the stack 'programs' path contains a space character. If the path also does not have an alternative short ('8 dot 3') name, the user is further warned of the implications for packages that make use of the GNU project's configure shell script.

New modules System.Info are added to src/windows and src/unix to re-export getShortPathName from the System.Win32.Info module of the Win32 package in the case of Windows and do nothing in the case of Unix-like operating systems. This avoids the need for C preprocessor (CPP) directives in Stack.Config.

ChangeLog.md is updated, accordingly.

Tested on Windows 10 with 'programs' paths that do and do not contain space characters and, in the latter case, with paths that do and do not have '8 dot 3' names. Tested with paths that do not yet exist when stack is first run.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary. N/A - documentation was the subject of Fix #4726: Improve docs on stack's programs path, on Windows #4798.

Please also shortly describe how you tested your change. Bonus points for added tests! See above.

[snoyberg]

Note that the CI build failed:

stack> /home/vsts/work/1/s/src/unix/System/Info.hs:9:33: error:
stack>     Not in scope: type constructor or class ‘IO’
stack>   |
stack> 9 | getShortPathName :: FilePath -> IO FilePath

[snoyberg]

LGTM, thanks!