make 'oracles' feature backwards compatible with php 7.4 (#13)

adds implementations of `str_contains` and `str_starts_with` that are used if the functions are not already defined. this should enable 4get to work on both php 7.4 and php 8.*

my instance is currently on 7.4.33, and you can see the oracles working here:
[calc.php](https://4get.silly.computer/web?s=2%2B4*(2^2))
[encoder.php](https://4get.silly.computer/web?s=md5+hamburger)
[numerics.php](https://4get.silly.computer/web?s=0xf0f0f)
[time.php](https://4get.silly.computer/web?s=what+is+the+time+right+now+tz%253Aest)

Reviewed-on: lolcat/4get#13
Co-authored-by: cynic <admin@cynic.moe>
Co-committed-by: cynic <admin@cynic.moe>

oracles/base.php

@@ -21,4 +21,16 @@ abstract class oracle {
 		return "";
 	}
 }
+// backwards compatibility
+if (!function_exists('str_starts_with')) {
+	function str_starts_with($haystack, $needle) {
+		return strncmp($haystack, $needle, strlen($needle)) === 0;;
+	}
+}
+if (!function_exists('str_contains')) {
+	function str_contains($haystack, $needle) {
+		return strpos((string)$haystack, (string)$needle) !== false;
+	}
+}
+
 ?>

oracles/calc.php

@@ -84,6 +84,13 @@ class calculator extends oracle {
 			}
 		}
 
+		// no implicit multiplication
+		for ($i = 0; $i < count($tokens) - 1; $i++) {
+			if ($tokens[$i][0] == "n" && $tokens[$i+1] == ["g", "("]) {
+				return "";
+			}
+		}
+
 		//strategy:
 		// traverse to group open (if there is one)
 		//  - return to start with the internals