Add support for nested declarations in Firebird procedural logic

[lukaseder]

In Firebird, we currently cannot use this kind of pattern:

create().begin(
    declare(i),
    i.set(1),

    declare(j),
    j.set(2),

    insertInto(t).columns(a, b).values(i, j)
).execute();

Firebird allows variable declarations only at the very beginning of an EXECUTE BLOCK AS statement, and that statement cannot be nested, see: https://stackoverflow.com/q/60654899/521799

We could probably re-write the entirety of the anonymous block, declaring all variables up front, and aliasing variables in case of ambiguities (which would also solve T-SQL's lack of block scope: #8314)

Some variable declarations are emitted by emulations, such as the indexed for loop, which currently doesn't work in Firebird because of the necessity of declaring a loop variable.

[lukaseder]

This has been fixed as #11366