Save command “Validate Syntax” with TextMate 2

#ignore
Change file name of command “Validate Syntax”

The filename for a command should be equal to its name inside the
bundle menu. This makes it easier to spot the file for a certain
command.

 #ignore
Improve otuput of “Validate Syntax”

The command now uses the selected web theme. Before this change it just
printed the error messages on a blank HTML page.
Use filename instead of path in “Validate Syntax”

This makes reading error messages easier. Before this change long
absolute file paths took some attention away from the important parts
of the error messages (line number, actual error).