Implements dedicated snapshots for Python Workers.

[dom96]

This PR implements dedicated memory snapshot support in Python Workers. These dedicated snapshots are intended to be returned via the validator and then stored in the WorkerBundle.

Run down of features this PR implements:

• new compatibility flag marked experimental which enables code paths to generate and load dedicated snapshots.
• new exceptions PythonRuntimeError and PythonUserError to help us distinguish between internal errors in our runtime code and user code errors Implements error types for Python runtime and PyodideEntrypointHelper. #4746
• dynamic libraries are now loaded from the vendored packages directory when necessary
• hiwire custom serialiser/deserialiser for global JS objects that our internal code references with a user-facing error message if their code attempts the same at the top-level of their worker
• collection of the snapshot at the very end, to ensure the full top-level execution is captured
• type for pyodide_entrypoint_helper Implements error types for Python runtime and PyodideEntrypointHelper. #4746

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[hoodmane]

The worst case I can think of for this is:

from js import Proxy, Object

p = Proxy.revocable(Object.new(), Object.new())
p.revoke()
q = p.proxy
del p

but this seems to mess up Pyodide's code too.

[hoodmane]

Opened upstream PR to fix:
pyodide/pyodide#5832

[hoodmane]

Suggested change to test this new flag against all current workerd tests:

diff --git a/build/python_metadata.bzl b/build/python_metadata.bzl
index 82b812a37..b8385e6b6 100644
--- a/build/python_metadata.bzl
+++ b/build/python_metadata.bzl
@@ -80,7 +80,10 @@ def _make_bundle_version_info(versions):
         if name != "development":
             entry["id"] = _bundle_id(**entry)
         entry["feature_flags"] = [entry["flag"]]
-        entry["feature_string_flags"] = [entry["enable_flag_name"]]
+        string_flags = [entry["enable_flag_name"]]
+        if entry["pyodide_version"] != "0.26.0a2":
+            string_flags.append("python_dedicated_snapshot")
+        entry["feature_string_flags"] = string_flags
         if "packages" in entry:
             entry["packages"] = entry["packages"]["info"]["tag"]
         _add_integrity(entry)

[hoodmane]

What happens if this is turned on with 0.26.0a2? Presumably we should bail out early?

[dom96]

Suggested change to test this new flag against all current workerd tests:

Even better I think I can use impliedByAfterDate to enable this compat flag if the 0.28 flag is enabled

[hoodmane]

Looks great, thanks!