Allow custom `RHINO_NPM` command (support `bun` and `pnpm`)

[kamilzyla]

Changes

This PR is a copy of #496, rebased onto main and with review suggestions applied. I couldn't edit the PR directly as it's in a forked repository; @nbbn is on sick leave till the end of the week and we don't want to wait (release 1.6 is underway).

How to test

1. Install npm alternative of your choice, e.g. bun or pnpm. Alternatively, create a simple wrapped-npm script (make it executable and place in your PATH):

   #!/bin/sh
   echo "Wrapper!"
   exec npm "$@"

2. Run e.g. rhino::lint_js() in a Rhino project. It should use npm by default.
3. Set RHINO_NPM to the command of your choice and try rhino::lint_js() again. You can do it by running R like this: RHINO_NPM=bun R.

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (9631c74) 17.82% compared to head (da8256c) 17.73%.

:exclamation: Current head da8256c differs from pull request most recent head 514a762. Consider uploading reports for the commit 514a762 to get more accurate results

Files	Patch %	Lines
R/node.R	0.00%	15 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #549 +/- ## ========================================== - Coverage 17.82% 17.73% -0.09% ========================================== Files 9 9 Lines 404 406 +2 ========================================== Hits 72 72 - Misses 332 334 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.