perf(syntax): avoid array spread in PathExpression.original getter

The getter on `PathExpression` in `legacy-interop.ts` is read once or
more per path node visit by every Ember template plugin. Its previous
form allocated a fresh array on every call:

    return [this.head.original, ...this.tail].join('.');

Replace with a direct head return for the empty-tail case and a string
concat for the rest:

    const head = this.head.original;
    return this.tail.length === 0 ? head : head + '.' + this.tail.join('.');

No behaviour change — both forms return the same string for every
(head, tail). See PR description for the equivalence proof and the
before/after measurements.

Author: johanrd

packages/@glimmer/syntax/lib/v1/legacy-interop.ts

@@ -48,7 +48,8 @@ export function buildLegacyPath({ head, tail, loc }: PathExpressionParams): ASTv
     head,
     tail,
     get original() {
-      return [this.head.original, ...this.tail].join('.');
+      const head = this.head.original;
+      return this.tail.length === 0 ? head : `${head}.${this.tail.join('.')}`;
     },
     set original(value: string) {
       let [head, ...tail] = asPresentArray(value.split('.'));