matrix_apply_linter() fails with interweaved comments

As fixed in #2822, `matrix_apply_linter()` errors under the following test:

```r
expect_lint(
  trim_some("
    apply(x, 2, #comment
    mean)
  "),
  ".",
  matrix_apply_linter()
)
# Error in `lint(file, ...)`: Linter `linter()` failed in '/tmp/Rtmpzq66vo/file2ed877bdfc561':
# Caused by error in `parse()`:
# ! <text>:2:0: unexpected end of input
# 1: apply(x,2,#commentmean)
#    ^
```