webassembly/variants/pyscript: Add pyscript variant.

This commit adds a pyscript variant for use in https://pyscript.net/.

The configuration is:
- No ASYNCIFY, in order to keep the WASM size down and have good
  performance.
- MICROPY_CONFIG_ROM_LEVEL_FULL_FEATURES to enable most features.
- Custom manifest that includes many of the python-stdlib libraries.
- MICROPY_GC_SPLIT_HEAP_AUTO to increase GC heap size instead of doing a
  collection when memory is exhausted.  This is needed because ASYNCIFY is
  disabled.  Instead the GC collection is run at the top-level before
  executing any Python code.
- No MICROPY_VARIANT_ENABLE_JS_HOOK because there is no asynchronous
  keyboard input to interrupt a running script.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/variants/pyscript/manifest.py

@@ -0,0 +1,27 @@
+require("abc")
+require("base64")
+require("collections")
+require("collections-defaultdict")
+require("copy")
+require("datetime")
+require("fnmatch")
+require("functools")
+require("gzip")
+require("hmac")
+require("html")
+require("inspect")
+require("io")
+require("itertools")
+require("locale")
+require("logging")
+require("operator")
+require("os")
+require("os-path")
+require("pathlib")
+require("stat")
+require("tarfile")
+require("tarfile-write")
+require("time")
+require("unittest")
+require("uu")
+require("zlib")

ports/webassembly/variants/pyscript/mpconfigvariant.h

@@ -0,0 +1,3 @@
+#define MICROPY_CONFIG_ROM_LEVEL                (MICROPY_CONFIG_ROM_LEVEL_FULL_FEATURES)
+#define MICROPY_GC_SPLIT_HEAP                   (1)
+#define MICROPY_GC_SPLIT_HEAP_AUTO              (1)

ports/webassembly/variants/pyscript/mpconfigvariant.mk

@@ -0,0 +1,3 @@
+JSFLAGS += -s ALLOW_MEMORY_GROWTH
+
+FROZEN_MANIFEST ?= variants/pyscript/manifest.py