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.

doc/site/modules/io/stdin.markdown

@@ -16,6 +16,12 @@ Defaults to `false`.
 
 Sets raw mode on or off.
 
+### **isTerminal**
+
+Returns `true` if Stdin is connected to a "TTY". This is true when the user is
+running Wren in an interactive terminal, and false if it its input is coming
+from a pipe.
+
 ### **readByte**()
 
 Reads one byte of input from stdin. Blocks the current fiber until a byte has