Introduce configuration file support

[vinistock]

While Rubydex is a framework that can be used to build all sorts of tools, in most cases we want consistency in the base analysis.

For example, imagine you have an LSP and a linter based on Rubydex, but because they each configure the framework differently, you get different results in the editor vs in the CLI. That would be a pretty poor experience.

I believe we can provide an easy way to ensure consistency while still maintaining flexibility so that the odd case can load custom configuration files for a random tool.

Implementation

I recommend reviewing per commit:

1. Allow loading configuration from a file. This adds the infrastructure to read, parse and apply a configuration file, hooking it up to the graph. See the example configuration file below for the format
2. Exposes load_config in the Ruby API
3. Adds rubydex.toml default configuration for this repo itself

# rubydex.toml
# Comments
single_value = true

multi_value = ["one", "two"]

[something.nested]
enabled = false

[st0012]

The implementation looks good but I'm not sure about the format we use for .rubydex.
I personally have NEVER used the INI format in any tools before, nor have seen it being used in the Ruby ecosystem. So I think using it would add additional friction to properly configure Rubydex for users.
I think yml is still the best choice for user adoption. toml will be fine for me too.

[vinistock]

Switched to TOML according to our discussions.

[st0012]

In the PR description, we still mention .rubydex and INI format. Let's update it
Also, should we use .rubydex.toml? Is it intentional to drop . prefix with switching to toml?

[vinistock]

I removed the leading dot intentionally since we now have a file extension. Just rubydex (instead of .rubydex) felt odd, but rubydex.toml seems fine to me.