chore(developer): Error: 4054 Statement extended is not currently supported in context message is unclear

[mcdurdin]

Reported at https://community.software.sil.org/t/error-4054-statement-extended-is-not-currently-supported-in-context/8571

This error message actually means 'you cannot use keys in the context part of the rule` (and will never be able to because it's a logical impossibility!)

So two things:

• It is suggesting future support for the concept with the word 'currently'
• 'extended' is internal babble exposed to the unsuspecting user

It would also help to reference the documentation at https://help.keyman.com/developer/language/guide/rules

At the same time as updating the error message, let's add a clarifying para on that page: the documentation already helps to differentiate between the different parts of a rule, but given how the understanding of 'a sequence of keystrokes' vs 'existing context + a keystroke' is so common, we should probably also add clarification of that specific idea.

[mcdurdin]

This was fixed by #9908 -- new error message in .kmx compile phase that means this one can no longer be emitted (because this was raised in .js compile phase, after .kmx)