php 7.4 fixes

Author: ArtemGoutsoul

src/CoffeeScript/Lexer.php

@@ -654,7 +654,7 @@ function interpolate_string($str, array $options = array()) // #{0}
 
     while ( isset($str{++$i}) )
     {
-      $letter = $str{$i};
+      $letter = $str[$i];
 
       if ($letter === '\\')
       {

src/CoffeeScript/Parser.php

@@ -3014,7 +3014,7 @@ function yy_r186(){ $this->_retvalue = yy('Op', $this->yystack[$this->yyidx + -1
 #line 3014 "/var/www/coffeescript-php/grammar.php"
 #line 294 "/var/www/coffeescript-php/grammar.y"
     function yy_r192(){
-  if ($this->yystack[$this->yyidx + -1]->minor{0} === '!') {
+  if ($this->yystack[$this->yyidx + -1]->minor[0] === '!') {
     $this->_retvalue = yy('Op', substr($this->yystack[$this->yyidx + -1]->minor, 1), $this->yystack[$this->yyidx + -2]->minor, $this->yystack[$this->yyidx + 0]->minor);
     $this->_retvalue = $this->_retvalue->invert();
   }

src/CoffeeScript/Scope.php

@@ -98,7 +98,7 @@ function declared_variables()
     {
       if ($v['type'] === 'var')
       {
-        if ($v['name']{0} === '_')
+        if ($v['name'][0] === '_')
         {
           $temp_vars[] = $v['name'];
         }