Support custom cell executions

[mxsdev]

Allows running cells with custom languages. Provides new program cell annotation, with which the cell will run.

Requires runme with stateful/runme#302

Depends on #590

[duvanmonsa]

LGTM

[sourishkrout]

Let's talk this thru today. I'd like us to provide more clarity that this, in fact, works like a #! shebang. We can decide to do that in a follow-up PR (likely more UX than logic).

Outside of that something like /usr/bin/env python3 does not work and errors will be "swallowed". Can we improve that please. Using env as a way to allow multi argument shebangs is commonplace.

[mxsdev]

Let's talk this thru today. I'd like us to provide more clarity that this, in fact, works like a #! shebang. We can decide to do that in a follow-up PR (likely more UX than logic).

Agreed. I used "program" as a placeholder (it fits with the internal naming structure), but I presumed we would be working it out more.

Outside of that something like /usr/bin/env python3 does not work and errors will be "swallowed". Can we improve that please. Using env as a way to allow multi argument shebangs is commonplace.

This is a bit complicated since we need to distinguish the program path from the arguments (this is probably a limitation of the syscall responsible for spawning a new process). Some paths can contain spaces, like "C:\Program Files...". For now I can use a simple regexp on whitespace.

But eventually we would need to (in descending order of difficulty):

1. Do some basic shell parsing (maybe a use case for shlex)
2. Wrap program execution in a bash shell (makes determining if the program exists [to create error window indicating unsupported language] difficult)
3. Regexp/split on whitespace (breaks filepaths with spaces)
4. Provide a separate input field for arguments (if i recall correctly, arguments are parsed by the system/shell program, so i don't think we need to worry here)

I think I can get this working with (1), maybe in a follow-up PR.

EDIT: looks like shebang line doesn't support spaces in path, so this would be extra functionality. Definitely interested in your thoughts @sourishkrout

[mxsdev]

@sourishkrout Just pushed changes that supports arguments in the program. One pretty massive issue, though: annotations don't allow spaces :/ not sure what to do about that.

I also fixed error handling. You should now get the following error popup if you don't provide a program, and the language is not supported:

Unable to automatically execute cell. To execute this cell, set the "program" field in the configuration foldout!

And the following error popup if the user-provided program does not exist:

Unable to locate program "<program>"

Happy to work out UX here 👍

[sourishkrout]

👍 LGTM, just some minor things. Feel free to change & merge.

[sourishkrout]

I'll do that before releasing 👍