libghostty: reduce Wasm stack reservation

Zig default's Wasm stacks to 1MB. Change it to 128 KB instead.

This removes 896 KiB from every Wasm instance's initial linear memory
reservation. That means that simply _loading_ `ghostty-vt.wasm` is down
this much.

Through various workload benchmarks of real terminal snapshots,
artificial worst case full ascii, full styled, full emoji, full mixed,
etc. workloads, I wasn't able to get a stack to go above 17 KB, so 128 KB
is VERY generous. Lets start here.

src/build/GhosttyLibVt.zig

@@ -58,6 +58,12 @@ pub fn initWasm(
     // There is no entrypoint for this wasm module.
     exe.entry = .disabled;
 
+    // Default Zig stack size in Zig 0.16 is 1MB. Lower to 128 KB since
+    // this has to be preallocated up front in Wasm linear memory. Peak
+    // stack under various artificial loads is 17 KB at the time of this
+    // comment but lets do 128KB to be safe. We can lower later.
+    exe.stack_size = 128 * 1024;
+
     // Zig's WASM linker doesn't support --growable-table, so the table
     // is emitted with max == min and can't be grown from JS. Run a
     // small Zig build tool that patches the binary's table section to