Reference substring with substr and mid

[theodesp]

According to the docs mid and substr are synonyms to substring:

https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_mid
https://dev.mysql.com/doc/refman/8.0/en/string-functions.html#function_substr

I will be willing to implement this.

[erizocosmico]

Feel free to take it if you want!

[theodesp]

#548

[theodesp]

Merged