Add "simple" strings

[Chimaine]

Currently, strings in MTS are enclosed by double-quotes " and support interpolation.

However, often interpolation is neither necessary nor wanted, and extending interpolation support could lead to more reserved characters and escaping.

To try and keep strings simple if they need to be, MTS should support "simple" strings enclosed by single-quotes ' that do not offer any kind of interpolation.

See Groovy's differentation between Strings and GStrings.