Support method-chaining for builtins to reduce need for custom-functions

[SebTM]

Currently it appears for me that for each non-trivial case, you might need a custom-function (or I miss something) I would have plenty of use-cases for method-chaining on builtin-functions e.g.

type PageData struct {
	Releases []struct {
		Version string `pagser:"div.c-page-details > div->textSplit(' ', true)->eq(3)"`
	} `pagser:"section.c-panel div.o-grid div.u-mb-16"`
}

What do you think? Would this be technically doable?