Handle escape sequences in strings (#7)

This should fix #6

corpus/expressions.txt

@@ -160,6 +160,16 @@ find "`dirname $file`" -name "$base"'*'
       (string (simple_expansion (variable_name)))
       (raw_string))))
 
+=========================================
+Strings containing escape sequence
+=========================================
+
+echo "\"The great escape\`\${var}"
+
+---
+
+(program (command (command_name (word)) (string)))
+
 =========================================
 Variable declaration: declare & typeset
 =========================================

grammar.js

@@ -269,7 +269,7 @@ module.exports = grammar({
       '"'
     ),
 
-    _string_content: $ => /[^"`$]+/,
+    _string_content: $ => /([^"`$]|\\.)*/,
 
     array: $ => seq(
       '(',

src/grammar.json

@@ -976,7 +976,7 @@
     },
     "_string_content": {
       "type": "PATTERN",
-      "value": "[^\"`$]+"
+      "value": "([^\"`$]|\\\\.)*"
     },
     "array": {
       "type": "SEQ",