Enable webr::eval_js() to return other types of R object

[georgestagg]

With this, the webr::eval_js() function can now return other types of R object, not just scalar integers. Returned JavaScript objects are converted to R objects using the RObject generic constructor, and specific R object types can be returned by invoking the R object constructor directly in the evaluated JavaScript.

Examples:

> webr::eval_js("123 + 456")
[1] 579

> webr::eval_js("Math.sin(1)")
[1] 0.841471

> webr::eval_js("(new Date()).toUTCString()")
[1] "Wed, 18 Sep 2024 09:39:54 GMT"

> webr::eval_js("new RList({ foo: 123, bar: 456, baz: ['a', 'b', 'c']})")
$foo
[1] 123

$bar
[1] 456

$baz
$baz[[1]]
[1] "a"

$baz[[2]]
[1] "b"

$baz[[3]]
[1] "c"