[wpiutil] StringExtras: Add substr() (#3742)

Unlike std::string and std::string_view, this substr() allows a start
greater than the length of the string, in which case an empty string
is returned.  This matches llvm::StringRef behavior.

cscore/src/main/native/cpp/HttpCameraImpl.cpp

@@ -208,7 +208,7 @@ wpi::HttpConnection* HttpCameraImpl::DeviceStreamConnect(
     if (wpi::trim(key) == "boundary") {
       value = wpi::trim(wpi::trim(value), '"');  // value may be quoted
       if (wpi::starts_with(value, "--")) {
-        value = value.substr(2);
+        value = wpi::substr(value, 2);
       }
       boundary.append(value.begin(), value.end());
     }