(BKR-1656) Fixes for cat and file_exist? on PSWindows

[sebastian-miclea]

These changes are needed in order to have the same output when these methods are used on different platforms. The implementation for unix is slightly different and had a different return values.
Having the same return values for both unix and windows implementation will reduce the need to check which os you're running on when writing beaker tests.

file_exist? had a different output compared to the unix implementation (which returns bool). It actually returned the position of where it found the the match.

2.5.3 :001 > "true" =~ /true/
 => 0
2.5.3 :002 > "true" =~ /rue/
 => 1

cat had different output due to the fact that execute does not wrap the output in an Response object, but exec does.

[mihaibuzgau]

Not taking into consideration the exit code of if exists seems that it does not impact the functionality.
Tests:
With directory that exists:

C:\Users\mitza>if exist C:\users echo true
true

C:\Users\mitza>echo %errorlevel%
0

With file that exists:

C:\Users\mitza>if exist C:\users\desktop.ini echo true
true

C:\Users\mitza>echo %errorlevel%
0

With directory or file that does not exist:

C:\Users\mitza>if exist C:\notexists echo true

C:\Users\mitza>echo %errorlevel%
0

relative path:

C:\Users\mitza>if exist notexists echo true

C:\Users\mitza>echo %errorlevel%
0

Characters that are not allowed in windows paths:

C:\Users\mitza>if exist <invalidwinpath echo true
< was unexpected at this time.

C:\Users\mitza>echo %errorlevel%
0