spikes: six v3 spikes (V8 harness, WASI shell, git, builder, http hook, esbuild)

Throwaway prototypes for Sausage v3. Plan and decisions:
~/research/sausage-v3/PLAN.md. Results and evidence: the spike READMEs.

spikes/s2-shell/process.wit

@@ -0,0 +1,6 @@
+package s2:process;
+interface host {
+    spawn: func(request: string) -> u32;
+    exchange: func(pid: u32, input: list<u8>, eof: bool, close-output: bool) -> tuple<u32, bool, u32, list<u8>, list<u8>>;
+}
+world shell { import host; }