A million variations exist. I'm looking for a programming-language-agnostic concept. The closest you'll find are templating languages like Handlebars
Delimited continuations? Though I'm not totally clear what you're asking for. Semantics or syntax?
Syntax. Delimited continuations would be a similar concept but for semantics
Function arguments are unfilled holes. So are interface declarations.
Both are (fairly) programming language agnostic, but in the context of an editor, would become specific. The editor would have to know _how_ to represent them in the code’s language.