`lsp.config[]` resolves an LSP config the first time it is called, and
returns the cached result on subsequent calls.
The change in #37571 added an extra call to `lsp.config[]` which will
resolve the config *before* the server is added to `_enabled_configs`,
meaning the result is discarded. That means configs will be needlessly
resolved again once `lsp_enable_callback` fires for the first time. That
includes an additional `loadfile()` call which is relatively expensive
and can have unexpected side effects.
Avoid this by storing the result of the initial call to `lsp.config[]`
in `_enabled_configs` so the config is not resolved a second time once
`lsp_enable_callback` is called for the first time.
eb90f5d9e3
