Add Pointer::starts_with and Pointer::ends_with

[asmello]

Long due! These are quite useful.

I initially named them is_suffix and is_prefix, but it turns out that's confusing to read (which is the prefix/suffix of which?). So I went with the std choice for the str counterparts.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.2%. Comparing base (b423489) to head (6503a68).
Report is 2 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/pointer.rs	98.2% <100.0%> (-0.1%)	⬇️