When using `true` as the value of a configuration option, the option is
configured to use default values. For example, if a user configures
virtual text to include the source globally (using
vim.diagnostic.config) and a specific namespace or producer configures
virtual text with `virt_text = true`, the user's global configuration is
overriden.
Instead, interpret a value of `true` to mean "use existing settings if
defined, otherwise use defaults".