LNumber.String() use strconv instead of fmt.Sprint

[MrParano1d]

You must post issues only here. Questions, ideas must be posted in discussions.

• [x] GopherLua is a Lua5.1 implementation. You should be familiar with Lua programming language. Have you read Lua 5.1 reference manual carefully?
• [x] GopherLua is a Lua5.1 implementation. In Lua, to keep it simple, it is more important to remove functionalities rather than to add functionalities unlike other languages . If you are going to introduce some new cool functionalities into the GopherLua code base and the functionalities can be implemented by existing APIs, It should be implemented as a library.

Please answer the following before submitting your issue:

1. What version of GopherLua are you using? : v1.1.1
2. What version of Go are you using? : go1.21.0 linux/amd64
3. What operating system and processor architecture are you using? : Win11 with WSL2
4. What did you do? : use tostring on a number
5. What did you expect to see? : use go formatting function for the datatype
6. What did you see instead? : fmt.Sprint

[MrParano1d]

Created PR. This will increase the performance in high traffic applications.