Substitution syntax in manifest template strings

At present, template strings in a Stripes Connect manifest can recognise and substitute sequences of the form:

• ?{name} – replaced with the URL query parameter name
• :{name} – replaced with the URL path component name

(And we will in future also be able to substitute from the React component state and/or props – see STRIPES-123.)

But sometimes we need to do more sophisticated things, dependent on whether or not the named parameter has a defined value. For example, to implement sorting, we want to include the sortby keyword in the CQL query only when the sort URL query parameter is defined.

To support this, we should implement fallbacks as in Bash, the Bourne Again Shell. See the Parameter Expansion section of its manual. For example, ?{sort:+sortby} should expand to the constant string sortby when the sort URL query parameter is defined, and nothing at all when it is not.