feat: convenience init for string-based TblTableRef

Sources/AndroidXML/TblTableRef.swift

@@ -13,6 +13,19 @@ public struct TblTableRef: CustomStringConvertible {
 		self.ident = ident
 	}
 	
+	/// Convenience constructor for string-based references.
+	/// Must conform to format `"@PPTTEEEE"`.
+	/// Throws an exception if string does not starts with `"@"` or cannot resolve hex-string into `UInt32`.
+	public init(_ stringRef: String) throws {
+		guard stringRef.hasPrefix("@") else {
+			throw AXMLError("A table reference must start with @")
+		}
+		guard let idx = UInt32(stringRef[(stringRef.index(after: stringRef.startIndex))..<stringRef.endIndex], radix: 16) else {
+			throw AXMLError("Invalid table reference format")
+		}
+		self.ident = idx
+	}
+	
 	/// `ident` as hex string
 	public var asHex: String { ident.hex(8) }