Replace "key" terminology with "index" terminology

[andyferris]

Our indexing API has a mixture of terminology, such as getindex, setindex!, keys and haskey. This issue suggest simplifying some of this, as a continuation of #25057.

Basically, the idea is to replace our "key" terminology with "index" terminology, for consistency and to reduce the number of words used to describe the same thing. After discussion, it seems that "index" makes some sense for arrays and dictionaries, while "key" seems wrong for arrays, so we'd unify these in the direction of "index". We might consider the following:

Current	"Index" terminology
getindex	getindex
setindex!	setindex!
keys	indices (in v1.0 not v0.7??)
haskey	hasindex (#25105)
keytype	indextype
KeySet	IndexSet

@StefanKarpinski can you please clarify whether prior discussions count as triage or not (i.e. apply the correct label / milestone). In particular, the above table includes a couple extra (obvious) items for consistency. Thanks!

[tkluck]

Just got reminded of this PR when I updated #27749 .

If we replace "key" by "index" (or vv), we should also consider "element" vs "value" (e.g. eltype vs valtype). I have no opinion either way, but thought it made sense to record that here.

[brenhinkeller]

I think we can call this "not planned" given the elapsed time and how breaking it would be

Nowadays there seems to be convention that indices are for AbstractArrays and keys are for collections where things may have names instead of just numbers