Support translating some string functions when they have a char parameters

[roji]

Following dotnet/efcore#34999

[ChrisJollyAU]

I must just tidy something up, then I will do a PR for you on this later this weekend

[ChrisJollyAU]

I noticed you havent implemented tests like IndexOf_with_constant_starting_position`, I can sort that out as well as part of this. I did the same fix for sqlite