Add the invert option for the ledc module

[pjsg]

• [x] This PR is for the dev branch rather than for the release branch.
• [x] This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• [x] I have thoroughly tested my contribution.
• [x] The code changes are reflected in the documentation at docs/*.

The latest version of the IDF adds support to invert the output sense of the ledc channel. This is useful if you are driving an active low device (such as an LED connected to VCC)