Improve handling of copilot_node_command

[timhugh]

A little history:

1. Support list/non-executable for copilot_node_command #498 identified that commands like "mise x node@lts -- node" could not be used for the copilot_node_command config because of excessive validation. This was resolved in Relax validation for copilot_node_command #527
2. Naturally, this broke a different use case identified in Could not determine Node.js version #538 -- now paths to node including a space no longer worked due to the use of vim.split. This was resolved in Fixes regression where node_command contained spaces #541

Unfortunately, that fix undid the previous fix, so this PR aims to resolve both:

1. I added tests for the get_execute_command and get_node_version functions of the nodejs module to prevent regression
2. copilot_node_command can now be configured two ways:
   • as a string: copilot_node_command = "/path to/node"
   • as a table: copilot_node_command = { "mise", "x", "node@lts", "--", "node" }

[AntoineGS]

Very happy with this initiative, I agree tests will be quite beneficial seeing the back and forth on this issue too.

[timhugh]

Okay, great! I did some test cleanup.

I'm pretty new to mini.test and I suspect there might be a more idiomatic way to structure the tests (maybe using sets and stubbing in the pre/post hooks instead of the stub methods?), but at least this covers a lot of cases

[AntoineGS]

Looks good, but I have a ton of failing tests which I do not think were caused by this but something on the LSP-side.
I'll wait until I've figured that out before merging just to have less code movement while I figure that out.

[AntoineGS]

Ok the issue resolved itself, all green!