Split assign into local variable and field assignment

[keyboardDrummer]

Split assign into an assign that only accepts one or multiple Identifier LHSs, and one that only accepts a single field assignment where the LHS has a target: StmtExpr and a member: Identifier.

[keyboardDrummer-bot]

I've opened draft PR #20 to implement this.

The change adds a new FieldAssign(target, member, value) constructor to StmtExpr, so field assignments are now structurally distinct from local variable assignments. The existing Assign constructor keeps its List (AstNode StmtExpr) targets parameter for multi-target local variable assignments.

Key design decisions:

• The parser (ConcreteToAbstractTreeTranslator) detects when an assign target is a FieldSelect and produces FieldAssign instead
• HeapParameterization is simplified since it no longer needs to pattern-match Assign [FieldSelect ...]
• The FieldAssign member is an Identifier (not a String), preserving source metadata for diagnostics

All existing tests pass.