Implement nullable analysis for collection literals

[cston]

See references to this issue in code.

This issue is blocked on a design decision from LDM: dotnet/csharplang#7626

• Element analysis (PR Collection expressions: analyze nullability of element assignments #70192)
• Re-inference (PR Collection expressions: method type re-inference #71214)
• Checking conversion of elements to iteration type for collection initializer types
• Spread analysis (PR Collection expressions: nullable analysis of spread element expression #74686)
• Checking constraints on the Create method
• Analysis of Add methods during re-inference

Relates to test plan #66418

[jcouv]

Assigned to @cston for the remaining two work items (spreads and an additional check to iteration type)