Add type check to theme_set()

[teunbrand]

This PR aims to fix #3117.

It adds a type check to theme_set(), that is all it does.

Examples (backtraces omitted):

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

# Fine
theme_set(theme_bw())

# Oops forgot parentheses
theme_set(theme_bw)
#> Error in `theme_set()`:
#> ! `new` must be a <theme> object, not a function.

# Plain wrong input
theme_set("foo")
#> Error in `theme_set()`:
#> ! `new` must be a <theme> object, not the string "foo".

^{Created on 2023-03-29 with reprex v2.0.2}

[thomasp85]

LGTM, but please update the snapshots to include the new test