feat: make string be external references host

Author: xepozz

src/main/kotlin/com/github/xepozz/php_opcodes_language/language/parser/PHPOp.bnf

@@ -67,7 +67,7 @@ parameter ::= IDENTIFIER
 
 string_literal ::= TEXT
 {
-    implements=["com.intellij.psi.PsiLanguageInjectionHost"]
+    implements=["com.intellij.psi.PsiLanguageInjectionHost" "com.intellij.psi.PsiLiteralValue"]
     extends="com.github.xepozz.php_opcodes_language.language.psi.impl.PHPOpStringLiteralBaseImpl"
 }
 
@@ -84,4 +84,4 @@ live_range_statements ::= (live_range_statement | EOL)*
 
 live_range_statement ::= NUMBER COLON live_range_range LPAREN live_type RPAREN
 live_range_range ::= NUMBER DASH NUMBER
-live_type::= IDENTIFIER (SLASH IDENTIFIER)?
+live_type::= IDENTIFIER (SLASH IDENTIFIER)?

src/main/kotlin/com/github/xepozz/php_opcodes_language/language/psi/impl/PHPOpStringLiteralBaseImpl.kt

@@ -7,7 +7,7 @@ import com.intellij.lang.ASTNode
 import com.intellij.psi.LiteralTextEscaper
 import com.intellij.psi.PsiLanguageInjectionHost
 
-abstract class PHPOpStringLiteralBaseImpl : PHPOpStringLiteral, PHPOpElementImpl, PsiLanguageInjectionHost {
+abstract class PHPOpStringLiteralBaseImpl : PHPOpStringLiteral, PHPOpElementImpl {
     constructor(node: ASTNode) : super(node)
 
     override fun getPresentation() = PresentationData(text, null, getIcon(0), null)
@@ -22,4 +22,6 @@ abstract class PHPOpStringLiteralBaseImpl : PHPOpStringLiteral, PHPOpElementImpl
     }
 
     override fun createLiteralTextEscaper() = LiteralTextEscaper.createSimple(this)
+
+    override fun getValue(): String = text
 }