Replace deprecated ArrayListUnmanaged with ArrayList

std.ArrayListUnmanaged is now std.ArrayList in Zig 0.15.
The old managed ArrayList is deprecated as std.array_list.Managed.

src/lexer.zig

@@ -128,8 +128,8 @@ pub const Lexer = struct {
     pos: usize,
     line: usize,
     column: usize,
-    indent_stack: std.ArrayListUnmanaged(usize),
-    tokens: std.ArrayListUnmanaged(Token),
+    indent_stack: std.ArrayList(usize),
+    tokens: std.ArrayList(Token),
     allocator: std.mem.Allocator,
     at_line_start: bool,
     current_indent: usize,