Fix multiline smart-knit

[ElianHugh]

What problem did you solve?

Closes #1403. Smart knitting was failing for multi-line custom knits because the filepath argument was being put on a newline (seemingly due to a linebreak). E.g.

[VSC-R] foo.Rmd process started
==> (function(input, ...) {
  rmarkdown::render(
    input,
    output_file = paste0(
      xfun::sans_ext(input), '-', Sys.Date(), '.html'
    ),
    envir = globalenv()
  )
})
('/run/media/elian/Main/Documents/Coding/R/enumr/foo.Rmd')

Trimming the whitespace from the knit param appears to fix this issue.

How can I check this pull request?

Smart knit a document with the following frontmatter:

---
knit: |
  (function(input, ...) {
    rmarkdown::render(
      input,
      output_file = paste0(
        xfun::sans_ext(input), '-', Sys.Date(), '.html'
      ),
      envir = globalenv()
    )
  })
---

This will succeed with this PR, but will fail otherwise. The output file should be something like foo-2024-02-23.html

[renkun-ken]

LGTM