Support cli functions in extract_glued_symbols()

[Bisaloo]

# glue::glue passes
lintr::lint(
  text = "foo <- function() { x <- 1; glue::glue('{x}') }",
  linters = lintr::object_usage_linter()
)

# cli functions lint
lintr::lint(
  text = "foo <- function() { x <- 1; cli::cli_h1('{x}') }",
  linters = lintr::object_usage_linter()
)
#> <text>:1:21: warning: [object_usage_linter] local variable 'x' assigned but may not be used
#> foo <- function() { x <- 1; cli::cli_h1('{x}') }
#>                     ^

# This is valid cli usage
x <- 1
cli::cli_h1('{x}')
#> 
#> ── 1 ───────────────────────────────────────────────────────────────────────────

^{Created on 2023-10-31 with reprex v2.0.2}

[MichaelChirico]

Didn't realize how highly requested this is!

One major issue is that {cli}'s mark-up syntax is much richer than {glue}'s, and there's currently no equivalent for extracting the variables besides custom logic we'd have to maintain & keep in sync:

r-lib/cli#750

In the short term, we could just treat cli exactly like glue, at least capturing simple glue-like interpolations, would that be useful enough as a first step?