[ENHANCEMENT] Support indexing of the form `a["i" => 1, ..]`

[mtfishman]

Support EllipsisNotation.jl indexing with ..:

using NamedDimsArrays
a = nameddims(randn(4, 4, 4), ("i", "j", "k"))
a["i" => 1, ..] # Equivalent to `a["i" => 1, "j" => :, "k" => :]`
a[.., "i" => 1] # Same thing, order doesn't matter

That would be very convenient notation for truncated tensor SVD, which would become:

u, s, v = svd(a, ("i",))
α = only(intersect(dimnames(u), dimnames(s)))
r = 1:3
u[α => r, ..], s[α => r, α => r], v[α => r, ..]

[mtfishman]

This should be easier to implement once implement #13.