Extend JsLIGO nested update syntax

This issue is part of the broader issue : #1597

This is a follow-up to the enabling of shorter syntax for nested record updates :

!2309 (merged)

The above MR allowed p.a.b.c = 42 (nested record updates).

We would like to enable also :

p.a[3] = 42; // Nested record *and tuples* accesses
p.a["b"] = 42; // Alternative syntax for record access