Add configuration options for third-party formatters

[andyw8]

Closes #1775

To properly support additional formatters, we need to change how formatters are selected when using VS Code.

Currently we use an enum with fixed set of choices. We could change this to a text field, but that result in a poorer user experience.

Another option is to add the additional formatters to the enum list.

Although we generally try keep ruby-lsp itself unaware of specific addons, in this case it seems a worthwhile trade-off.

With the new choices being added, there is some additional information we want to show the user. There are two ways we could do this:

Using enumDescriptions:

Using enumLabels:

[andyw8]

We'll co-ordinate with @searls to determine the release version for the work being done in standardrb/standard#630.

[st0012]

I'm not sure about maintaining this list while we don't have any control on the community addons. For example, ruby-lsp-rubyfmt is likely not working with the recent Ruby LSP releases as it doesn't use the latest addon interface. Can we instead ask users to choose auto and focus on letting the server pick the right formatter?

[st0012]

@vinistock mentioned that there are cases where it's not possible to purely rely addons & server to figure out the formatter. For example, if rubocop decided to become a Ruby LSP addon too and register the rubocop formatter the same way standard does, then Ruby LSP can't tell if the user wants to use standard or rubocop formatter by itself.

As an alternative, I'd like to propose a different solution here: for options that are not maintained by us, we make it extra clear and provide minimal context. So instead of:

• Standard (need v0.37 or newer of the standard gem)
• Rubyfmt (needs the ruby-lsp-rubyfmt gem)

We simply show:

• Standard (supported by community addon)
• Rubyfmt (supported by community addon)

And we use the same format for any community formatter addons in the future, which will set a clear expectation to the users and save us the need to update descriptions later.

[andyw8]

I'm good with that approach.

[andyw8]

@vinistock are you in agreement?

[vinistock]

Yeah, I'm good with that 👍