Experimentally add *heterogeneous* try blocks

148725 moved the default to being homogeneous; this adds heterogeneous ones back under an obvious-bikeshed syntax so people can experiment with that as well.

Essentially resolves 149025 by letting them move to this syntax instead.

tests/pretty/try-blocks.rs

@@ -0,0 +1,6 @@
+//@ pp-exact
+//@ edition: 2024
+
+#![feature(try_blocks, try_blocks_heterogeneous)]
+
+fn main() { try { Some(1)? }; try bikeshed Result<u32, ()> { 3 }; }