feat: Add double-quoted string constants with escapes

Resolves #20.

trans/statics.stx

@@ -184,16 +184,19 @@ constructor was trivial:
 
 Now typing literals is straightforward:
 ```statix
+{! "\git docs/statix_works:trans/statics.stx" extract:
+  start: '(.*ty_Ex.Int.*\s*)'
+  stop: '/. ../'
+!}
+```
   **/
 
-  /** md */
   ty_Ex(Int(_))       = INT().
   ty_Ex(LitString(_)) = STRING().
+  ty_Ex(EscString(_)) = STRING().
   ty_Ex(e@Stream())   = STREAM().
-  /* **/
 
   /** md
-```
 
 Finally we get to the binary operators, and here we use the pattern found in
 recent versions of the