Behavior of `aphaleia-npx` currently differs from other formatters by
triggering an error when formatter is missing (causing a message to be
displayed in minibuffer and an error buffer to be created).

This PR adds a test to check if command is on user's PATH before
executing it in order to make `aphaleia-npx` silently ends when the
formatter command is not found at project and system level.

* `apheleia-npx` would use an incorrect path for the Yarn PnP ESM
loader.
* `apheleia-npx` did not correctly guard against word splitting.
* `apheleia-npx` was sometimes not able to find formatters in a Yarn PnP
project if there was also a node_modules folder at the root of the
project. Unfortunately, many tools (including
[Prettier](https://github.com/prettier/prettier/issues/13032)) will
create a cache folder in `node_modules` even in Yarn PnP projects. The
presence of any `node_modules` folders are irrelevant when a `.pnp.cjs`
file is present.

---------

Co-authored-by: Radon Rosborough <radon@intuitiveexplanations.com>

Gets all non-elisp dependencies into a single directory,
`scripts/formatters`

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This adds support for formatters installed locally in project
directories via yarn 2's "zero install" [pnp
mode](https://yarnpkg.com/features/pnp).

It's quite similar to the support for formatters installed locally in a
project's `node_modules` via npm, and leverages the `npx` symbol, so
existing formatter definitions should work without modification.

This checks for a `.pnp.cjs` file (expected in the project root for yarn
pnp projects), then looks for a yarn executable, and checks the version
of yarn to make sure it supports pnp. If that works, we just push
`"yarn"` onto the front of `command`.

I've only tested this with a locally installed `prettier.js`. It's very
much a works-for-me draft, I'm putting in a PR to make sure this is a
workable approach before going any further with it.

---------

Co-authored-by: Radon Rosborough <radon@intuitiveexplanations.com>