refactor: Use npx jest as default jest_cmd

[SirWrexes]

• Slightly better start perf: No need to climb the directory tree
• More flexible: Either runs the locally installed version or asks to install it temporarily*
• Jest users (should) always have NPM installed, and as such, have npx available.

*A temporary install through npx still requires a config file present somewhere in the current working directory or above in the tree.
A default config file could fix that, but I think it should be the user's choice to set it up or not.
However, it would be nice to have an option to set the path to a global config file, and maybe recycle the old tree-climbing functions to find a local config, and then use the default if none is found... But then that wouldn't work with Jest configs from inside package.json. 😕

[SirWrexes]

Woops, it seems my formatter aggressively shifted all indentation there.

[SirWrexes]

I've got this use-case where I'm using ts-jest in a mono-repo and Jest is not picking up the TS configuration for one of the modules of the repo.
I wanted to be able to set a per-project config that would let me override jest_cmd with pnpm $(git rev-parse --show-toplevel)/relative/path/to/module test so I've added 9474264.

I'm using nvim-projectconfig and have a simple lua file for my project containing vim.g.jest_cmd = that long command thing and it works like a charm !