One of the steps in #23328
Before there were 3 different but similar functions: dict/Dict/mergeinto
The code was just copied & pasted, no test.
This PR defines a new stable `dict` function, it covers all the 3 old
functions behaviors, only +160 -171
Future developers do not need to think about or guess the different dict
functions, just use one: `dict`
Why use `dict` but not `Dict`? Because there are far more `dict` than
`Dict` in code already ......
