input configuration to pass input as stdin on startup

This adds a new configuration `input` that allows passing either raw
text or file contents as stdin when starting the terminal.

The input is sent byte-for-byte to the terminal, so control characters
such as `\n` will be interpreted by the shell and can be used to run
programs in the context of the loaded shell.

Example: `ghostty --input="hello, world\n"` will start the your default
shell, run `echo hello, world`, and then show the prompt.

src/termio/Thread.zig

@@ -146,6 +146,8 @@ pub fn threadMain(self: *Thread, io: *termio.Termio) void {
         // have "OpenptyFailed".
         const Err = @TypeOf(err) || error{
             OpenptyFailed,
+            InputNotFound,
+            InputFailed,
         };
 
         switch (@as(Err, @errorCast(err))) {
@@ -165,6 +167,24 @@ pub fn threadMain(self: *Thread, io: *termio.Termio) void {
                 t.printString(str) catch {};
             },
 
+            error.InputNotFound,
+            error.InputFailed,
+            => {
+                const str =
+                    \\A configured `input` path was not found, was not readable,
+                    \\was too large, or the underlying pty failed to accept
+                    \\the write.
+                    \\
+                    \\Ghostty can't continue since it can't guarantee that
+                    \\initial terminal state will be as desired. Please review
+                    \\the value of `input` in your configuration file and
+                    \\ensure that all the path values exist and are readable.
+                ;
+
+                t.eraseDisplay(.complete, false);
+                t.printString(str) catch {};
+            },
+
             else => {
                 const str = std.fmt.allocPrint(
                     alloc,