Add isTerminal and previousInChain getters to AstNode

[TwitchBronBron]

Expression chains are stored inverted in the AST, so walking .parent can't tell when you've left one expression and landed in an unrelated enclosing one:

print doSomething(a.b)

PrintStatement
└─ CallExpression                  "doSomething(a.b)"
   ├─ callee: VariableExpression   "doSomething"
   └─ args[0]: DottedGetExpression "a.b"     ← its own expression
      └─ obj: VariableExpression   "a"

Walking .parent from a gives a.b, then the CallExpression — but that call isn't part of a.b, it's the thing a.b sits inside.

Each node in a chain spans a whole prefix of the source, so a node's previous step is the next-shorter prefix:

print a.b[2].c(9)
      ─                  a
      ───                a.b
      ──────             a.b[2]
      ────────           a.b[2].c
      ─────────────      a.b[2].c(9)

Which parent property holds the child is what decides membership, not node type. So each chaining node declares its previous step:

// DottedGetExpression
public get previousInChain() { return this.obj; }     // `a.b` -> `a`

// CallExpression
public get previousInChain() { return this.callee; }  // `a.b()` -> `a.b`, not the args

and isTerminal() walks up then back down — if it doesn't land on itself, nothing chains onto it:

public isTerminal(): boolean {
    return this.parent === undefined || this.parent.previousInChain !== this;
}

This is not parent reversed: args and index values have a parent, but are never anything's previousInChain, which is exactly what makes them boundaries.

Two members total. Nodes that chain onto nothing (literals, binary expressions, groupings) inherit previousInChain === undefined and are terminal by default, so boundaries need no code.

print a.b.c(1)
// a.b.c(1)  terminal - the whole expression
// a.b.c     no       - a.b.c(1) chains onto it
// a.b       no       - a.b.c chains onto it
// a         no       - a.b chains onto it
// 1         terminal - an argument, so its own expression

What changed

• AstNode: previousInChain getter and isTerminal()
• previousInChain overrides on CallExpression, CallfuncExpression, DottedGetExpression, IndexedGetExpression, XmlAttributeGetExpression, NamespacedVariableNameExpression, NewExpression

Verified — 3108 passing (upstream master is 3084; +24 tests covering each boundary type: call args, index values, groupings, binary/unary operands, array & AA literals, ternary, null-coalescing, template interpolations, set-statement operands). tsc --noEmit and npm run lint clean. Purely additive against master.