feat(errors): register UnhandledMatchError as a builtin Error subclass

[chadmandoo]

What

UnhandledMatchError — PHP 8's error thrown when a match expression has no matching arm and no default — was not a registered builtin, so a program that references it (including the common default => throw new UnhandledMatchError(...) idiom) failed to type-check.

function classify(int $n): string {
    return match (true) {
        $n < 0   => "neg",
        $n === 0 => "zero",
        default  => throw new UnhandledMatchError("no arm for " . $n),
    };
}
try { classify(5); } catch (\UnhandledMatchError $e) { echo $e->getMessage(); }

How

Register UnhandledMatchError as a builtin class extending Error (the class-name list + the class_map insert in declarations.rs) and include it in the Error-family signature fixup (exception.rs), mirroring TypeError / ValueError / ArithmeticError.

Test

test_unhandled_match_error_throw_and_catch — newable, throwable from a match default arm, catchable by FQN, getMessage() round-trip. Byte-parity vs PHP 8.5.

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