Generated code with '()' caused parsing errors.

[emosheeep]

https://github.com/sxzz/unplugin-ast/blob/fb31eeb8ca343d5b67cfa364ab5dba9812d61eaa/src/core/transform.ts#L69

I tried to return an AST node to change code content, but I found that the generated code would be wrapped with parentheses, which would cause parsing error.

e.g.

(var wrappedVar = 'test';) // Error! Unexpected token at ....