TList<T> now exposes a `default` array property, so elements can be read
and written with the familiar subscript syntax instead of .Get(i):
L[0] := 100; // L.SetItem(0, 100)
WriteLn(L[0] + L[1]); // L.Get(i)
Adds TList<T>.SetItem (a pointer-target store, so the compiler's ARC
discipline releases the old element and retains the new for managed T)
and the property Items[AIndex]: T read Get write SetItem; default.
Also fixes the generic-instance clone path in the semantic analyser: it
copied every property-decl field except IsDefault, so a default property
declared on a generic template (TList<T>) lost its default flag on
instantiation and List[i] did not resolve. The clone now carries
IsDefault.
Verified List[i] read, write, and polymorphic element dispatch
(List[i].Area through a base-typed element) on both backends. Adds
TE2ETListTests.TestRun_TList_DefaultProperty_{ReadWrite,Polymorphic}.
Note: this makes the stdlib depend on the `default` directive (added in
the previous commit), so the pre-release bootstrap binary is refreshed to
a default-aware stage-2 in a follow-up.
42ca9d7535