coalesce operators '??' and '?=' now accepts function definitions too

parser_test.go

@@ -134,7 +134,7 @@ func TestParser(t *testing.T) {
 		/* 113 */ {`import("test-funcs.expr"); (double(3+a) + 1) * two()`, int64(34), nil},
 		/* 114 */ {`x ?? "default"`, "default", nil},
 		/* 115 */ {`x="hello"; x ?? "default"`, "hello", nil},
-		/* 116 */ {`x ?? func(){}`, nil, errors.New(`[1:14] the right operand of a coalescing operation cannot be a function definition`)},
+		/* 116 */ {`y=x ?? func(){4}; y()`, int64(4), nil},
 		/* 117 */ {`x ?= "default"; x`, "default", nil},
 		/* 118 */ {`x="hello"; x ?= "default"; x`, "hello", nil},
 		/* 119 */ {`@x="hello"; @x`, nil, errors.New(`[1:3] variable references are not allowed in top level expressions: "@x"`)},