[Feature request] Case-insensitive string enums/aliases

The World of Warcraft Lua API features an enum for frame positioning that is like:

"LEFT" | "RIGHT" | "TOP" | ...etc

The problem is: it's not case sensitive. Some prefer writing "left" or "Left" instead of "LEFT". Technically even "lEfT" would be accepted.

The problem if we just add all styles of writing the enum it will look bad in autocomplete etc.

So the request is to add a way to say that upper vs. lowercase doesn't matter for an enum.

该 issue 未指定文件、测试或实现入口。首先定位 Lua language server 中的 enum 处理和自动补全生成，然后确定在不列出每种拼写的情况下，应如何表示不区分大小写的 enum。Done 应包含所请求的行为，并保留干净的自动补全结果。