Add functions to handle string prefixes and suffixes

Introduces the following functions:
* `hasPrefix $prefix $s`
* `hasSuffix $suffix $s`
* `trimPrefix $prefix $s`
* `trimSuffix $suffix $s`

These call the corresponding `strings.{Has,Trim}*` functions with the arguments flipped to better facilitate function pipelining for template usage (e.g. `$val := split $input $sep | first | trimSuffix $suffix`).
See also the reversal of the variables in `range` for Go templates v. Go itself.