Add task to create makeshift JavaScriptCore "docs"

Actually just grepping for exported names, but it works well enough for
a start.

c/ton/Makefile

@@ -2,3 +2,6 @@ JSC_CFLAGS = $(shell pkg-config javascriptcoregtk-4.0 --cflags --libs)
ton: *.c
	clang $(JSC_CFLAGS) main.c -o $@
jsc-funcs:
	grep -Rh JS_EXPORT /usr/include/webkitgtk-4.0/JavaScriptCore | sed 's/^JS_EXPORT //' | grep -v '^#' > $@