fix: don't mutate the defaultSettings object passed to SettingsContainer

[Yanhu007]

Fixes #1627

Problem

SettingsContainer stores a direct reference to the defaultSettings object:

constructor(defaultSettings: T) {
    this.#defaultSettings = Object.assign({}, defaultSettings); // ✅ copy
    this._settings = defaultSettings;                           // ❌ direct reference
}

When set() is later called, Object.assign(this._settings, settings) mutates the original object. This means a shared defaultCompilerOptions object gets extra properties (lib, configFilePath) after creating a Project, and those leak into subsequent Project instances.

Fix

Create a shallow copy for _settings, matching the existing behavior of #defaultSettings:

this._settings = Object.assign({}, defaultSettings);

One-character fix (Object.assign({}, ...) instead of direct assignment).