Unknown json-rpc requests should get an error response, not be ignored

## Information

VIM version: 2:8.2.2434-3+deb11u1
Operating System: Debian

## What went wrong

For full description, and instructions on how to reproduce, please see: https://github.com/LuaLS/lua-language-server/issues/2318

While lua-language-server shouldn't need to, it does attempt to call _workspace/configuration_:

```json
{
"id":1,
"jsonrpc":"2.0",
"method":"workspace/configuration",
"params":{
"items":[
{
"scopeUri":"file:///tmp/luals",
"section":"Lua"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.associations"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.exclude"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.semanticHighlighting.enabled"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.acceptSuggestionOnEnter"
}
]
}
}

{
"id":2,
"jsonrpc":"2.0",
"method":"workspace/configuration",
"params":{
"items":[
{
"scopeUri":"file:///tmp/luals",
"section":"Lua"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.associations"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.exclude"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.semanticHighlighting.enabled"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.acceptSuggestionOnEnter"
}
]
}
}

{
"id":3,
"jsonrpc":"2.0",
"method":"workspace/configuration",
"params":{
"items":[
{
"scopeUri":"file:///tmp/luals",
"section":"Lua"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.associations"
},
{
"scopeUri":"file:///tmp/luals",
"section":"files.exclude"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.semanticHighlighting.enabled"
},
{
"scopeUri":"file:///tmp/luals",
"section":"editor.acceptSuggestionOnEnter"
}
]
}
}
```

There are no responses from ale. While it is silly to attempt the call to a client lacking the capability, the Language Server Protocol Specification [mandates](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#requestMessage) that:

> Every processed request must send a response back to the sender of the request.

From context it appears this _must_ is to be considered a MUST as defined by [rfc2119](https://www.rfc-editor.org/rfc/rfc2119).