Please add function preview feature

[wzhran]

In RStudio, when you have loaded a function, it will show in the environment panel. Then, you can click the function name to open it in a new window. It is really convenient to look into the details of the function or even modify it. Please develop a similar one for Positron.

[jennybc]

Related to #2945

[juliasilge]

Yes! @wzhran I added your use case in #2945 (comment), if you'd like to give a 👍 or add any additional info.

Thank you for the report!