Add an 'is string' type check #22

Problem

A definition can check that a field is a set, multiset, list or int, but there's no way to say "this field is a single value, not a collection". Several Redux definitions already write S is string (the DFA and NFA definitions, for example), but string isn't a keyword. The grammar reads it as a name (a TERM), so is string doesn't do the type check its author meant.

Wanted

• is string as a built-in type check, next to int in coll.g4 and ConstraintDescriptor. It accepts a single value (a word such as kitten or New York) and rejects a set or list, with a SpadeParseException like the other type checks (is int and is set constraints don't check their type #10 / fix(grammar): make is int, is set and is list check their type #15).
• Docs in DefinitionString.md, and tests next to TypeConstraintTests.cs.
• Optional, could be a separate issue: quoted values. Today a value can't contain , { } ( ), so free text like "hello, world" can't be part of an instance. Something like "hello, world" would let a field hold any text. Redux's Lossless Data Compression needs this. Nothing else does yet.

Redux context

Redux is replacing its plain-English instance grammars with real SPADE definitions, so that the planned per-part instance editor can split every problem's instance (ReduxISU/Redux#851). Edit Distance, (x, y) where x and y are strings, needs is string, and so do the existing DFA and NFA definitions. #18 should report this type for each name like the others.