Change the case_insensitive argument to case_sensitive

[kelnage]

When adding the ability to configure how the conversion backend works with queries that do not require case insensitivity (the default for Sigma rules), the variable to control this was named case_insensitive. However, this is a negative statement, which has lead to confusion for end-users and developers, so renaming it to case_sensitive should help alleviate this issue.

One concern about making this change is it would inconvenience any existing users, so we could look into creating a backwards compatibility approach of deprecating (but not disabling) the case_insensitive flag whilst pushing users to move to case_sensitive with a warn or similar (and produce an error if both are set in opposition). Doing so would make this a medium size task.