This patch adds 2nd class support for hooks using julia as the language.
pre-commit will install any dependencies defined in the hooks repo
`Project.toml` file, with support for `additional_dependencies` as well.
Julia doesn't (yet) have a way to install binaries/scripts so for julia
hooks the `entry` value is a (relative) path to a julia script within
the hooks repository. When executing a julia hook the (globally
installed) julia interpreter is prepended to the entry.

Example `.pre-commit-hooks.yaml`:

```yaml
- id: foo
  name: ...
  language: julia
  entry: bin/foo.jl --arg1
```

Example hooks repo: https://github.com/fredrikekre/runic-pre-commit/tree/fe/julia
Accompanying pre-commit.com PR: pre-commit/pre-commit.com#998

Fixes pre-commit#2689.