Make treeshaking handle "multiassignments"

[mischnic]

↪️ Pull Request

The treeshake function tried to remove the right side of assignment if expression was of this form (when nested in a sequence expression):

var $TJPF$export$default = ($TJPF$export$THING = ($TJPF$export$SOME = undefined, $TJPF$exports.SOME = $TJPF$export$SOME), $TJPF$exports.THING = $TJPF$export$THING);

(Don't think "multiassignments" are the official name)

Bug was introduced by myself in #2418 🙈
Part of #1984

💻 Examples

from react-transition-group:

exports.default = exports.EXITING = exports.ENTERED = exports.ENTERING = exports.EXITED = exports.UNMOUNTED = void 0;

var UNMOUNTED = 'unmounted';
exports.UNMOUNTED = UNMOUNTED;
var EXITED = 'exited';
exports.EXITED = EXITED;
var ENTERING = 'entering';
exports.ENTERING = ENTERING;
var ENTERED = 'entered';
exports.ENTERED = ENTERED;
var EXITING = 'exiting';
exports.EXITING = EXITING;

🚨  Property right of AssignmentExpression expected node to be of a type ["Expression"] but instead got null
    at Object.validate (.../parcel/node_modules/@babel/types/lib/definitions/utils.js:128:13)
    at Object.validate (.../parcel/node_modules/@babel/types/lib/validators/validate.js:17:9)
    at NodePath._replaceWith (.../parcel/node_modules/@babel/traverse/lib/path/replacement.js:194:9)
    at NodePath._remove (.../parcel/node_modules/@babel/traverse/lib/path/removal.js:51:10)
    at NodePath.remove (.../parcel/node_modules/@babel/traverse/lib/path/removal.js:30:8)
    at remove (.../parcel/packages/core/parcel-bundler/src/scope-hoisting/shake.js:109:25)
    at Array.forEach (<anonymous>)
    at Object.keys.forEach.name (.../parcel/packages/core/parcel-bundler/src/scope-hoisting/shake.js:30:65)
    at Array.forEach (<anonymous>)
    at treeShake (.../parcel/packages/core/parcel-bundler/src/scope-hoisting/shake.js:20:33)
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

after hoisting:

var $TJPF$export$default = ($TJPF$export$THING = ($TJPF$export$SOME = undefined, $TJPF$exports.SOME = $TJPF$export$SOME), $TJPF$exports.THING = $TJPF$export$THING);

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs