Add `language_servers` setting for customizing which language servers run (#10911)

This PR adds a new `language_servers` setting underneath the language
settings.


This setting controls which of the available language servers for a
given language will run.

The `language_servers` setting is an array of strings. Each item in the
array must be either:

- A language server ID (e.g., `"rust-analyzer"`,
`"typescript-language-server"`, `"eslint"`, etc.) denoting a language
server that should be enabled.
- A language server ID prefixed with a `!` (e.g., `"!rust-analyzer"`,
`"!typescript-language-server"`, `"!eslint"`, etc.) denoting a language
server that should be disabled.
- A `"..."` placeholder, which will be replaced by the remaining
available language servers that haven't already been mentioned in the
array.

For example, to enable the Biome language server in place of the default
TypeScript language server, you would add the following to your
settings:

```json
{
  "languages": {
    "TypeScript": {
      "language_servers": ["biome", "!typescript-language-server", "..."]
    }
  }
}
```

More details can be found in #10906.

Release Notes:

- Added `language_servers` setting to language settings for customizing
which language server(s) run for a given language.