feat: custom containers and GitHub alerts

[brenoepics]

This is a useful feature if users want custom containers with not only custom icons/labels but also custom colors or styles.
currently, they would have to disable the GitHub flavored plugin and enable it with new code to support it or add another containers plugin with custom containers only.

fixes #3591
fixes #3602

Usage Example:

Custom Container

You may set custom containers globally by adding the following content in site config:

// config.ts
export default defineConfig({
  // ...
  markdown: {
    container: {
      //...
      customContainers: {
        example: 'Example Label'
      }
    }
  }
  // ...
})

And add your custom style:

/* override.css */
.custom-block.example {
  border-color: var(--vp-c-brand-1);
  color: var(--vp-c-brand-2);
  background-color: var(--vp-c-brand-3);
}

.custom-block.example a,
.custom-block.example code {
  color: var(--vp-c-brand-1);
}

.custom-block.example a:hover,
.custom-block.example a:hover > code {
  color: var(--vp-c-brand-2);
}

.custom-block.example code {
  background-color: var(--vp-c-brand-1);
}