feat: add Stdin.isRaw property to query and set terminal raw mode on stdin

Add foreign static getter `isRaw` and setter `isRaw=(value)` to the Stdin class, backed by a new `isStdinRaw` boolean flag and `uv_tty_set_mode` calls. The getter returns the current raw mode state, while the setter initializes the stdin stream if needed and applies raw or normal mode only when stdin is a TTY. Also includes a test file verifying default false, reading bytes in normal mode, enabling raw mode, and reading subsequent bytes.

test/io/stdin/is_raw.wren

@@ -0,0 +1,28 @@
+import "io" for Stdin
+
+// Defaults to false.
+System.print(Stdin.isRaw) // expect: false
+
+// stdin: abcdefgh
+for (i in 1..4) {
+  System.print(Stdin.readByte())
+}
+// expect: 97
+// expect: 98
+// expect: 99
+// expect: 100
+
+Stdin.isRaw = true
+System.print(Stdin.isRaw) // expect: true
+
+for (i in 1..4) {
+  System.print(Stdin.readByte())
+}
+// expect: 101
+// expect: 102
+// expect: 103
+// expect: 104
+
+// TODO: This doesn't actually detect a visible difference between raw and
+// non-raw mode. Maybe add support to the test runner for writing non-printing
+// characters to stdin?