This removes the need to run ./scripts/code.sh first

Fixes #5587

As with #3649 it removes the need for global installation of gulp.
This was already the case for the windows, in ./scripts/code.bat.

Also added an exec to improve performance (which replaces the current process instead of spawning a subshell),
and changed the shebang to #!/usr/bin/env as this is best practice.

This reverts commit 37dbbb35.

#1523

# Conflicts:
#	src/vs/languages/html/common/htmlTags.ts