feat: support getters and setters in a class body

[marevol]

Stacked on #59.

Object literals took accessors and classes did not, so the class form was a syntax error:

class C { get g() { return 2; } }          // Expected ( but found g
class C { set s(v) { this._v = v; } }      // Expected ( but found s
class C { static get g() { return 3; } }   // Expected ( but found g

Plain and static methods already worked; only the accessor form was missing.

Parsing

The member loop now recognises get and set the way an object literal does, and reuses propertyGetterFunction and propertySetterFunction to read them, so the parsing rules — property name, empty getter parameter list, sloppy setter parameter — are the same in both places.

get and set stay usable as ordinary member names. What follows the keyword decides, as it does in an object literal: a property name makes it an accessor, a parameter list makes it a method. class C { get() {} } and static get() {} both still parse as methods.

Lowering

A method lowers to an assignment on the prototype, or on the class itself when static. An accessor is not expressible as an assignment, and unlike an object literal member there is no literal here to carry it, so it lowers to Object.defineProperty on the same target, with configurable: true and enumerable: false.

Defining one at a time is correct for a get/set pair: a descriptor only changes the fields it names, so a setter defined afterwards keeps the getter beside it.

Two things worth recording

Object.defineProperty is read from the global scope, so a class body evaluated where Object is shadowed would not find it. That is the same shape every ES6→ES5 lowering of accessors takes, and it adds no runtime helper.

An accessor is correctly non-enumerable. A method, on the prototype by plain assignment, is still enumerable, which ES6 says it should not be. That is pre-existing and not changed here.

Verification

./gradlew build testOptimistic testPessimistic

suite	before	after
test	666, 0 fail	666, 0 fail
testOptimistic	1718, 0 fail	1718, 0 fail
testPessimistic	1718, 0 fail	1718, 0 fail