Add Casio BASIC

## Language name
Casio BASIC

## URL of example repository
https://github.com/barneyhill/casio-simplex
https://github.com/AkzidenzGrotesk-py/CasioBasic
https://github.com/Dig03/CasioBASIC

## URL of syntax highlighting grammar
https://github.com/malagasonfire/prizmbasic

## Most popular extensions

`.cas` `.g1m` `.g2m` `.g3m` `.casiobasic` `.txt` `.`

## Detected language

`Text` or `Vim Help File` when its extension is `.txt`

## More informations
https://en.wikipedia.org/wiki/Casio_BASIC
https://community.casiocalc.org/topic/2448-casio-basic-tutorial/

Start by reviewing existing language additions in the Linguist repository and compare the listed Casio BASIC repositories, grammar, extensions, and detected-language values with those patterns. Done means Casio BASIC is represented consistently in Linguist and its common files are detected as intended, with the relevant tests passing.

Written by the indexing model from the issue text.