Use 'compatibility flag' language consistently

[irvinebroque]

refs EW-7514

• Previously we used the language "feature flag" and "compatibility flag" interchangeably
• This was ambiguous, since on Cloudflare and other platforms, "feature flag" refers to a separate concept and system, that requires internal action to enable.
• This PR updates all references to "feature flag" to use the "compatibility flag" language, to ensure that when people encounter error messages, it is clear that these are in reference to compatibility flags, as described here in the Workers docs.

[ohodson]

Thanks Brendan

When looking at this PR and existing code, there's also features.h (compatibility-flags.h?) and the FeatureFlags therein that should probably be aligned with this, but this is a cross repo change that doesn't need to be part of this change.

[ohodson]

The text here appears 3 times so could go into a convenience macro. Perhaps in features.h along the lines of:

#define COMPATIBILITY_NOTICE(Thing, FlagForThing)                                 \
  "To use the new " Thing " enable the " FlagForThing " compatibility flag."      \
  "Refer to the docs for more information: "                                      \
  "https://developers.cloudflare.com/workers/platform/compatibility-dates/#compatibility-flags"

[jasnell]

Yeah, I went with FeatureFlags for the new thing because of the existing language that was already there. I'd made a note to revisit it but it was a low priority.