Support slashy interpolated string

[bentsherman]

Close #49

Testing support for slashy interpolated strings, which can be useful for defining an interpolated regex without needing as many escapes.

TODO:

• provide an example that shows the benefit over a regular interpolated string

[bentsherman]

This case doesn't work:

id = 'SRA001'
println /${id}.fastq/

It gives Unexpected input: '<EOF>' at the end of the script

Adding parentheses is a workaround:

id = 'SRA001'
println(/${id}.fastq/)

But still the slashy string isn't being parsed correctly:

GStringExpression(2:9-2:21)
  ConstantExpression(2:11-2:20)

It is parsing the string contents as plain text rather than the ${id} expression.