Add Grain language support

cli/assets/templates/grain/.gitignore

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+*.wasm

cli/assets/templates/grain/Makefile

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+build:
+	grain compile --release --elide-type-info \
+		--import-memory --use-start-section --memory-base 8192 \
+		--initial-memory-pages 1 --maximum-memory-pages 1 \
+		--wasi-polyfill bindings/wasm4-wasi-polyfill.gr -I bindings \
+		app/main.gr -o cart.wasm
+
+clean:
+	find -name '*.wasm' | xargs rm

cli/assets/templates/grain/README.md

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+# Grain bindings for WASM-4
+
+Requires grain 0.6 or higher. Tested with Grain 0.6.3. For fast compiling, be sure to [build Grain from source](https://grain-lang.org/docs/getting_grain#Building-Grain-from-Source).
+
+For original template source, see:
+
+- https://github.com/ospencer/wasm4-gr
+
+Tips and source here are copied from there.
+
+## Tips to fit within 64k
+
+This limitation is part of the fun. Grain's extensive `Number` type brings in quite a bit of support code, so avoiding it (and standard libraries that depend on it) is the best way to keep the cartridge size small. This includes the math operations provided by `Pervasives`, like `+` and `==`, but conveniently, the WASM-4 API uses the `Uint8` and `Uint16` types. The `hello-world.gr` example is only 7k, and even with a number of sprites and a good chunk of music data, the `music.gr` example is 21k.
+
+Good luck and have fun!

cli/assets/templates/grain/app/main.gr

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+module HelloWorld
+
+from "wasm4" include Wasm4 as W4
+from "uint8" include Uint8
+use Uint8.{ (+), (-), (&), (>), (==) }
+
+let smiley = b"\xc3\x81\x24\x24\x00\x24\x99\xc3"
+
+let itsDangerous = "IT'S DANGEROUS\n TO GO ALONE!"
+
+let mut x = 76us
+let mut y = 76us
+
+let mut frame = 0us
+
+provide let update = () => {
+  if (frame == 108us) {
+    frame = 0us
+  }
+
+  // Load the gamepad
+  let gamepad = W4.gamepad1()
+
+  // Move the player location
+  if ((gamepad & W4._BUTTON_LEFT) > 0us) {
+    x -= 1us
+  }
+  if ((gamepad & W4._BUTTON_RIGHT) > 0us) {
+    x += 1us
+  }
+  if ((gamepad & W4._BUTTON_UP) > 0us) {
+    y -= 1us
+  }
+  if ((gamepad & W4._BUTTON_DOWN) > 0us) {
+    y += 1us
+  }
+
+  // Swap to color 2
+  W4.drawColors(2uS)
+  // Write the text
+  W4.text(itsDangerous, 24us, 50us)
+
+  // Swap to color 3
+  W4.drawColors(3uS)
+  // Draw the sprite
+  W4.blit(smiley, x, y, 8us, 8us, W4._BLIT_1BPP)
+
+  frame += 1us
+}