Fix our own implementation of “contains” with special characters in the pattern

Previously, “contains('foo\', '\')” would fail with an error, due to the
antislash not being escaped when calling the “regexp” function. Same for other
special characters in regular expressions (e.g. the dollar sign).

In particular, this bug would make Dynare unusable with Octave under
Windows (since commit e9d79796), because
Windows paths typically contain antislashes.