Add removeprefix/removesuffix to string

[ndmitchell]

Someone wanted to change libFoos.so to Foos so they did the (not unreasonable) x.lstrip("lib").rstrip(".so"), which looks like it does that, but completely doesn't (it returns Foo). I then change that to index slicing, in particular [:-len(extension)], but that fails in the corner case of an empty extension. These aren't unreasonable things to do, but the default Starlark operators are full of pitfalls.

Python 3.9 added string.removeprefixand string.removesuffix. I've added them to the Rust Starlark, since they are super helpful and avoid bugs. It would be great to add them to standard Starlark too.

[adonovan]

Seems reasonable. I'm amazed they took until 3.9 to appear.

[fmeum]

@adonovan Given these are already in Rust Starlark, what would I need to do to add them to the spec + Bazel? Happy to work on PRs as I would find these functions very useful.

[adonovan]

Thanks for volunteering. Could you send a PR to change to the spec adjacent to https://github.com/bazelbuild/starlark/blob/master/spec.md#string%C2%B7endswith, which we can review? Once that's approved the maintainers of the various implementations can add the function, which should be very straightforward. I can do the Go one and @brandjon can do Java.

[fmeum]

I submitted #212 as an update to the spec and have prepared bazelbuild/bazel#14824 to implement it in Java Starlark.