feat: adding .forRoot() method

[willmendesneto]

Please check if the PR fulfills these requirements

• The commit messages follow these
  guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other. Please describe:

If it is a Bugfix, please describe the root cause and what could have been done to prevent it…

What is the current behavior? (You can link to an open issue here, add screenshots…)

N/A

What is the new behavior?

Usage:

import { NgModule } from '@angular/core';
import { NgxSkeletonLoaderModule } from 'ngx-skeleton-loader';
// ... list of other app dependencies

import { AppComponent } from './app.component';
// ... list of other app components/modules

@NgModule({
  declarations: [AppComponent],
  imports: [NgxSkeletonLoaderModule.forRoot()],
  providers: [],
  bootstrap: [AppComponent],
})
export class AppModule {}

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No