Whole Project Linting

I've thought about how to handle whole project linting for years, and I think I finally have a good idea of how to handle it.

1. Allow an optional `project_command` key to be set for linters to define a command to explicitly use for running linters over an entire project.
2. Allow `project_root` to be defined for non-LSP linters for setting the `cwd` to use for a project command. We can make this optional for tools that don't require a specific work directory, and define that any configured root will be used as the `cwd` of a project command, such as through `ale_root`.
3. Discourage use of quickfix for storing the results for a single file at a time, and make errors for a whole project available in quickfix. Note in documentation that you probably can't use both at once.
4. Note in documentation that Language Server Protocol currently does not offer a way to get a list of all problems for an entire project.
5. Require an explicit command to be run to get problems for an entire project. There can be no automatic linting of an entire project.
6. Allow `project_command` and `callback` to be defined for language server linters, so they can separately launch an executable to find all errors. See Pyright for example.
7. We will define that `callback` may be called for either file-specific or project-wide errors if a linter is configured to permit project wide linting.
8. Note in documentation the subset of linters that specifically implement project-wide linting.
9. Print a warning when no linters enabled for a buffer support project-wide linting.

The implementation details are complicated, but we can implement this. It will be complex, but worth doing.

My recommendation for implementing how to get ALE to display project-wide problems across different buffers is to implement project wide collection of problems in its own file and define it to use the "other sources" API as if it were a separate plugin. That will prevent the engine code from being complicated by project-wide linting, and will make the addition of project wide linting nearly zero added cost for users who don't use it.

We can make project wide linting populate quickfix while sending the problems per-buffer to ALE's usual rendering of problems.