Support templateContent and templatePath options

[bertdeblock]

Users can now use custom templates via 1 of 2 options:

1. Using the templateContent option:

// gember.config.js

export default {
  generators: {
    component: {
      templateContent: '<template>\{{yield}}</template>';
    },
  },
};

2. Using the templatePath option:

// gember.config.js

export default {
  generators: {
    component: {
      templatePath: './path/to/custom-component-template.gjs';
    },
  },
};