terminal: make stream processing infallible

The terminal.Stream next/nextSlice functions can now no longer fail.
All prior failure modes were fully isolated in the handler `vt`
callbacks. As such, vt callbacks are now required to not return an error
and handle their own errors somehow.

Allowing streams to be fallible before was an incorrect design. It
caused problematic scenarios like in `nextSlice` early terminating
processing due to handler errors. This should not be possible.

There is no safe way to bubble up vt errors through the stream because
if nextSlice is called and multiple errors are returned, we can't
coalesce them. We could modify that to return a partial result but its
just more work for stream that is unnecessary. The handler can do all of
this.

This work was discovered due to cleanups to prepare for more C APIs.
Less errors make C APIs easier to implement! And, it helps clean up our
Zig, too.

example/zig-formatter/src/main.zig

@@ -23,8 +23,8 @@ pub fn main() !void {
 
         // Replace \n with \r\n
         for (buf[0..n]) |byte| {
-            if (byte == '\n') try stream.next('\r');
-            try stream.next(byte);
+            if (byte == '\n') stream.next('\r');
+            stream.next(byte);
         }
     }
 

example/zig-vt-stream/src/main.zig

@@ -14,24 +14,24 @@ pub fn main() !void {
     defer stream.deinit();
 
     // Basic text with newline
-    try stream.nextSlice("Hello, World!\r\n");
+    stream.nextSlice("Hello, World!\r\n");
 
     // ANSI color codes: ESC[1;32m = bold green, ESC[0m = reset
-    try stream.nextSlice("\x1b[1;32mGreen Text\x1b[0m\r\n");
+    stream.nextSlice("\x1b[1;32mGreen Text\x1b[0m\r\n");
 
     // Cursor positioning: ESC[1;1H = move to row 1, column 1
-    try stream.nextSlice("\x1b[1;1HTop-left corner\r\n");
+    stream.nextSlice("\x1b[1;1HTop-left corner\r\n");
 
     // Cursor movement: ESC[5B = move down 5 lines
-    try stream.nextSlice("\x1b[5B");
-    try stream.nextSlice("Moved down!\r\n");
+    stream.nextSlice("\x1b[5B");
+    stream.nextSlice("Moved down!\r\n");
 
     // Erase line: ESC[2K = clear entire line
-    try stream.nextSlice("\x1b[2K");
-    try stream.nextSlice("New content\r\n");
+    stream.nextSlice("\x1b[2K");
+    stream.nextSlice("New content\r\n");
 
     // Multiple lines
-    try stream.nextSlice("Line A\r\nLine B\r\nLine C\r\n");
+    stream.nextSlice("Line A\r\nLine B\r\nLine C\r\n");
 
     // Get the final terminal state as a plain string
     const str = try t.plainString(alloc);