Add can and regex functions

[nadav-y]

based on #88 (comment)
Adding "can" (https://developer.hashicorp.com/terraform/language/functions/can) function, similar to try but returns bool

Adding "regex" (https://developer.hashicorp.com/terraform/language/functions/regex) similar to regexall, but return first found instead of a list of results

[nadav-y]

I've changed the implementation of can based on the examples provided