Require whitespace before as in to/downto labels

The `RE_TO_DOWNTO_AS_LABELS` rule previously used `\s*` between `(to|downto)` and `(as)`,
allowing zero spaces and accidentally matching identifiers like "toast".

Change the pattern to `\s+` so that an actual space is required, preventing spurious `as`
highlighting inside identifiers.

Author: mediremi

grammars/rescript.tmLanguage.json

@@ -21,7 +21,7 @@
           }
         },
         {
-          "match": "(to|downto)\\s*(as)",
+          "match": "(to|downto)\\s+(as)",
           "captures": {
             "1": {
               "name": "variable"