Merge `custom_parse` into `custom_func` and retire `custom_parse`?

[jf-tech]

Previously we created custom_parse because of limitation of return type of a custom_func must be string. Since we revamped custom_func type system because of javascript introduction, seems like there is no reason for custom_parse to exist anymore. custom_parse is simply just another custom_func with transformctx and idr.Node two arguments and an arbitrary interface{} return type.

Should we just deprecate and retire custom_parse?