Initial Flux language specification

Add the LL(1) context-free grammar (GRAMMAR.ebnf), token and syntax
reference (SYNTAX.md), LL(1) verification tool (ll1_check.py), and a
fibonacci example demonstrating the language.

examples/fibonacci.flx

@@ -0,0 +1,7 @@
+fn fibonacci(n: u8) -> u64 {
+    if n < 2 {
+        return n;
+    }
+
+    return fibonacci(n - 1) + fibonacci(n - 2);
+}