Support generic content entity tokens

Problem/Motivation

This is a follow-up from #2877378: Add token replacements for Media → .

Currently core defines custom tokens for some content entities (node.tokens.inc, user.tokens.inc, taxonomy.tokens.inc, comment.tokens.inc, etc).

The token contrib module provides generic entity tokens, including fields. That functionality could be moved to core and then eliminate the need of so many entity-specific tokens.

Proposed resolution

- Discuss the better approach to standardize content entity tokens in core
- Implement it

Remaining tasks

User interface changes

API changes

Data model changes