Rename `Feature` to `Experiment`

[justlevine]

What problem does this address?

Feature is an ambiguous term and easily confused with (lowercase f) feature. Since this plugin is still in the scaffold state we can avoid a ton of unnecessary future DX headaches by using a different name to differentiate our various modules.

What is your proposed solution?

In line with the name of the plugin, I suggest we rename all references of (capital F) Feature to Experiment.

This change would

• better aligns with the name of the plugin - AI Experiments not AI Features
• more accurately reflect that a top-level module can contain multiple (lowercase) features
• simplify communication (e.g. "To use the Post Excerpt auto generation feature, enable the Content Generation ~Feature~ Experiment and toggle Generate Post Excerpts ).

We have a limited window to implement this change where it costs us "nothing", and I am happy to pick it up once approved (not worth doing in advance bc of the inherent merge conflicts). As we start shipping features and people begin using this plugin, it starts to make less and less sence.

[jeffpaul]

Seems fine to me, though would like the larger scaffolding concerns to get worked out in #61 before we confirm/commit on this naming approach.

[JasonTheAdams]

Yeah, this makes sense to me, especially given the name. Let's make a PR for this after #61.