Expose backtrace filtering facilities

[wokalski]

When you write custom test DSLs you want to see expectation failures in your DSL code, not somewhere deep in a utility.
It'd be great to expose this filtering somewhere:
https://github.com/facebookexperimental/reason-native/blob/d3203eaeb05b49837fa85b1daed4b31390b64e27/src/rely/StackTrace.re#L125

[wokalski]

I didn't have time to properly test it but I couldn't figure out what's the exclusion rule really. It seemed to work once and didn't work the other time (when I moved more of the DSL to a separate file).

[wokalski]

Now that I think about it I know what was the issue. It proves this behavior is not enough 🤔