Enhance Alt-Enter to send multi-line expressions

**Is your feature request related to a problem? Please describe.**

The current behavior of the Alt-Enter shortcut will only send a single line at a time, unless in visual mode. Many common expressions span multiple lines and in interactive development it is convenient to write an expression and immediately send it to the interpreter. Right now, if one writes a multiline expression, one must

1. Write it
2. Exit insert mode
3. Enter visual mode
4. Highlight the whole expression
5. Alt-Enter

**Describe the solution you'd like**

Alt-Enter should find the beginning and end of the expression where the cursor is and send the entire expression. This should find the surrounding top-level `let`-binding, module definition, or the beginning and end of a top-level pipeline.

**Describe alternatives you've considered**

I am wondering if it is worth the time to try to write the 'find expression bounds' logic in vimscript and use that to bind a custom shortcut to do the actions described above. This feels like functionality that should be core to the plugin, though, and not the domain of user customizations.