Document bypassTypechecker` for contributors, and enable automatically where possible

[andyw8]

In normal use, when working on a codebase that uses Sorbet, Ruby LSP will defer to Sorbet's LSP for some features.

But when working on the Ruby LSP itself, we want to 'dogfood' it as we develop, so we avoid using Sorbet in the editor.

To override Ruby LSP's default behaviour, we have a bypassTypechecker setting. However, if this is not enabled, it can make it appear that some features are 'broken' when working on Ruby LSP itself. This is a confusing barrier for new contributors.

Instead, for VS Code users, we can detect if the Sorbet LSP extension is active, and automatically set bypassTypechecker.

For contributors using other editors, we should clearly document this setting.