Feature: Allow for configurable set of recognized lua file extensions

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Windows

### What is the issue affecting?

Other

### Expected Behaviour

It would be nice if the extension could recognize files as lua based on a set of configurable extensions.

### Actual Behaviour

Currently, it seems to only recognize files with `.lua` as their extension as lua files.

### Reproduction steps

Create a lua file with a different extension, open the file in VSCode. The editor has no idea the file is lua.

### Additional Notes

_No response_

### Log File

_No response_

Reproduce the behavior in the Visual Studio Code extension by opening a Lua file with a non-.lua extension and confirming it is not recognized. Trace the extension's file-language recognition and configuration entry points, then verify that users can configure a set of extensions and that those files receive Lua language support.

Written by the indexing model from the issue text.