sub-replace-expression (:s/.../\=...)

[dhnam]

Is your feature request related to a problem? Please describe.
I was looking for easy way to multiply number, and found out there's a way - using expression in substitute (:s).

For example, :s/\d\+/\=submatch(0)*5/ is valid command in vim - it multiplies number by five.

But it doesn't work in vsvim.

Describe the solution you'd like
It is feature supported in normal Vim, as written in vimhelp.

So implementing this could be solution.

Additional context
There was an issue about expression (#4383). It was not about expression in replace, but it was about expression register.

[J-Fields]

Most of what's needed for this is in #7920. I'll try to revive that PR soon.