Improve string parsing

Single and double quoted strings are now parsed as different entities.
This lays the ground for different escape sequences in both types of
strings.  Also it will allow the detection of questionable language
elements like non-working interpolation in a single quoted string.

grammar.js

@@ -69,8 +69,8 @@ module.exports = grammar({
   externals: $ => [
     $.qmark,
     $.selbrace,
-    $._fixed_string,
-    $._expandable_string,
+    $._sq_string,
+    $._dq_string,
     $._heredoc_start,
     $._heredoc_body,
     $._heredoc_end,
@@ -602,11 +602,22 @@ module.exports = grammar({
 
     // Literals (dynamic and static)
 
-    _quotedtext: $ => choice($.string, $.heredoc),
+    _quotedtext: $ => choice(
+      $.single_quoted_string,
+      $.double_quoted_string,
+      $.heredoc
+    ),
+
+    single_quoted_string: $ => seq(
+      "'",
+      repeat($._sq_string),
+      "'",
+    ),
 
-    string: $ => choice(
-      seq("'", repeat($._fixed_string), "'"),
-      seq('"', repeat(choice($._expandable_string, $.interpolation, $.escape_sequence)), '"'),
+    double_quoted_string: $ => seq(
+      '"',
+      repeat(choice($._dq_string, $.interpolation, $.escape_sequence)),
+      '"',
     ),
 
     heredoc: $ => seq(