Rename SystraceSection to TraceSection (#48383)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48383

## Changelog:
[Internal] -

This changes the name of `SystraceSection` class to `TraceSection`, the purpose being to make it Systrace/FBSystrace agnostic (and that it can be mapped to e.g. Perfetto instead).

It changes all the internal callsites to the RN code code, and also adds a shim include, `<cxxreact/SystraceSection.h`, for backward compatibility with the external callers for now (which will be addressed separately).

Reviewed By: javache

Differential Revision: D67621914

fbshipit-source-id: 337c63c45a7b075c6e00cfca67ecc06c298c94c0