Use base::is.expression() to determine expression-ness

[teunbrand]

This PR aims to fix r-lib/marquee#76.

Essentially rlang::is_expression() treats strings as expressions, which we don't need.
I couldn't find a reason in the commit history other than 'use more rlang functions' for this, so I hope I'm prancing around Chesterton's fence.
Reprex from the issue:

devtools::load_all("~/packages/test/gganimate/")
#> ℹ Loading gganimate
#> Loading required package: ggplot2
library(ggplot2)
library(marquee)

anim <- ggplot(economics, aes(date, unemploy)) + 
  geom_line() + 
  labs(title = "Unemployment") + 
  theme(plot.title = element_marquee()) + 
  transition_reveal(date)

# avoid hurting the poor eyes with windows png device
animate(anim, device = "ragg_png")

^{Created on 2025-06-23 with reprex v2.1.1}