Some inconsistencies when using Or matching

Typically, if `@capture(expr, match_1)` matches you would expect `@capture(expr, match_1 | match_2)` to always also match. This is however not the case. For example:

```julia
julia> @capture(:(begin; x = 2;y=3; end), begin body_ end)
true

julia> body
quote
#= REPL[2]:1 =#
x = 2
#= REPL[2]:1 =#
y = 3
end
```

matches, but the following

```julia
julia> @capture(:(begin; x = 2;y=3; end), begin body_ end | for i_ in iter_ forbody_ end)
false
```

does not. In addition, it the expression in the block only contains one expression, there will be a match:

```julia
julia> @capture(:(begin; x = 2; end), begin body_ end | for i_ in iter_ forbody_ end)
true

julia> body
:(x = 2)
```

This feels inconsistent to me.

Start at the @capture entry point and reproduce the two-expression and one-expression examples from the issue. Trace how the Or pattern handles begin blocks and add a regression test for both cases; done means the combined pattern matches whenever the standalone begin pattern matches.

Written by the indexing model from the issue text.