[Suggestion] Add ability to define an enum without using a table

### 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?

Annotations, Type Checking, Completion

### Suggestion

For existing codebases where enums are defined like this:
```
OPERATING_SYSTEM_WINDOWS = 1
OPERATING_SYSTEM_MAC = 2
KEY_A = 1
KEY_B = 2
```

it would be nice to be able to define all lines that follow (until the next blank line) as part of an enum without changing anything by introducing a table: i.e.
```
@enum OPERATING_SYSTEM
OPERATING_SYSTEM_WINDOWS = 1
OPERATING_SYSTEM_MAC = 2

@enum KEY
KEY_A = 1
KEY_B = 2
```

This would be convenient for type annotating when the values are passed as parameters and what-not. I'm not able to find any existing ways to designate these values as a "group" (or union) that maintains their use as global constants.

Start by locating the annotation, type-checking, and completion entry points that process global constants. Compare the proposed @enum blocks with existing annotation syntax and determine how enum membership should affect parameter typing while preserving global constant usage. Done means consecutive constants can be grouped until a blank line and are recognized consistently by annotations, type checking, and completion.

Written by the indexing model from the issue text.