Add OPTIONAL, OPT back to UPARSE

This adds OPTIONAL and OPT, but temporarily bridges over still tolerating
TRY, which will be deprecated after a while.

src/mezz/uparse.r

@@ -256,17 +256,17 @@ default-combinators: make map! reduce [
 
     === BASIC KEYWORDS ===
 
-    'opt combinator [
+    'try combinator [
         {If applying parser fails, succeed and return NULL; don't advance input}
         return: "PARSER's result if it succeeds, otherwise NULL"
             [any-value? pack?]
         parser [action?]
         <local> result'
     ][
-        fail "OPT combinator replaced in UPARSE by TRY"
+        fail "TRY combinator changed back to OPTIONAL (or abbreviate as OPT)"
     ]
 
-    'try combinator [
+    'optional combinator [
         {If applying parser fails, succeed and return NULL; don't advance input}
         return: "PARSER's result if it succeeds, otherwise NULL"
             [any-value? pack?]
@@ -2588,6 +2588,12 @@ default-combinators: make map! reduce [
 default-combinators.(tuple!): default-combinators.(word!)
 
 
+=== ABBREVIATIONS ===
+
+default-combinators.opt: default-combinators.optional
+default-combinators.try: default-combinators.optional  ; deprecated
+
+
 === COMPATIBILITY FOR NON-TAG KEYWORD FORMS ===
 
 ; !!! This has been deprecated.  But it's currently possible to get this