Square brackets ignore class field check

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

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Type Checking, Diagnostics/Syntax Checking

### Expected Behaviour

Classes check fields correctness regardless of access form class["bad"] and class.bad should both cause error

### Actual Behaviour


### Reproduction steps

Create class with fields. Access to non-existing field using `["field-name"]`.

### Additional Notes

_No response_

### Log File

_No response_

Reproduce the diagnostic in the Lua language server using a class field access written as class["field-name"], then compare it with class.field. Trace the type-checking and diagnostics path responsible for class field lookup; done means both access forms report nonexistent fields consistently.

Written by the indexing model from the issue text.