Handle nested unclosed template literal strings

tentative fix for rescript-lang#432 (rescript-lang#435)

Author: bobzhang

syntax/.gitignore

@@ -7,3 +7,4 @@ lib/*
 !lib/README.md
 .vscode/settings.json
  # each person has its own preferences
+ _build

syntax/src/res_scanner.ml

@@ -487,11 +487,11 @@ let scanTemplateLiteralToken scanner =
       | _ ->
         next scanner;
         scan ())
-    | ch when ch == hackyEOFChar ->
+    | ch when ch = hackyEOFChar ->
       let endPos = position scanner in
       scanner.err ~startPos ~endPos Diagnostics.unclosedTemplate;
       Token.TemplateTail(
-        (String.sub [@doesNotRaise]) scanner.src startOff (scanner.offset - 1 - startOff)
+        (String.sub [@doesNotRaise]) scanner.src startOff (max (scanner.offset - 1 - startOff) 0)
       )
     | _ ->
       next scanner;