add fallback and receive keywords

Author: taquangtrung

solidity-mode.el

@@ -110,6 +110,7 @@ Possible values are:
  "event"
  "error"
  "external"
+ "fallback"
  "for"
  "function"
  "if"
@@ -131,6 +132,7 @@ Possible values are:
  "private"
  "public"
  "pure"
+ "receive"
  "require"
  "return"
  "returns"