Fix lengthBytesUTF8 not a function (#255)

* Add lengthBytesUTF8 as an exported runtime method

* Also export printErr as a runtime method

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Co-authored-by: Austen Adler <agadler@austenadler.com>

rules.mk

@@ -18,6 +18,7 @@ $(BUILD)/$(BIN).html: $(OBJS)
 	@echo HTML $@
 	@$(CC) $(LDFLAGS) $(OBJS) $(LIBS) -o $@ \
 		-s ASYNCIFY=1 \
+		-s EXPORTED_RUNTIME_METHODS=lengthBytesUTF8,printErr \
 		-s EXPORTED_FUNCTIONS=_main \
 		--shell-file=$(TOP)/watch-library/simulator/shell.html