Handle alternate linux config directory paths for addons

[sparr]

Microsoft's build of VSCode uses ~/.config/Code on Linux. Popular alternate builds VSCodium and Code - OSS use different names for their config directories. And if a distro or user compiles with an uncommon name, it could be arbitrary.

This patch checks the three named variations, then all config dirs, then produces an error.

I also had it memoize the addons path rather than search for it 3+ times during startup (once for every instance of ${addons})

This fixes the problem I ran into in LuaLS/LLS-Addons#182

[sumneko]

I think addone should be sent from the client to the server, instead of having the server search for it. Custom options initialized through LSP or passed as startup parameters could be considered.

[sparr]

I think that would involve asking the extension's LuaClient.start() to pass context.globalStorageUri as a parameter to the language server binary. I got a bit lost trying to figure out how command line option handling works in the language server, though.

[sumneko]

https://github.com/LuaLS/vscode-lua/blob/2d28d6a804c48d5489cd7968e5208d818bfa06ba/client/src/languageserver.ts#L148-L157

Use initializationOptions may better

[sparr]

No longer necessary with #2893 merged