feat: add Stdin.isTerminal method to detect TTY connection

Add a new `isTerminal` static method to the Stdin class that returns `true` when
stdin is connected to a TTY (interactive terminal) and `false` when input comes
from a pipe. Implementation uses libuv's `uv_guess_handle` to check the stdin
descriptor. Includes documentation, foreign method declaration, module
registration, and a test expecting `false` since tests run non-interactively.

test/io/stdin/is_terminal.wren

@@ -0,0 +1,4 @@
+import "io" for Stdin
+
+// The tests aren't run in a terminal.
+System.print(Stdin.isTerminal) // expect: false