feat: allow customizing the alert type to admonitions directive mapping via the plugin options

[LuudJanssen]

This change allows you to customize the mapping of GitHub alert types to admonition directive names using the mapping option:

import { remark } from "remark";
import remarkDirective from "remark-directive";
import remarkGithubAdmonitionsToDirectives, {
  DEFAULT_MAPPING,
  DirectiveName,
  GithubAlertType,
  type AlertTypeMapping,
} from "remark-github-admonitions-to-directives";

const mapping: AlertTypeMapping = {
  ...DEFAULT_MAPPING,
  [GithubAlertType.IMPORTANT]: DirectiveName.WARNING,
};

const processor = remark()
  .use(remarkGithubAdmonitionsToDirectives, { mapping })
  .use(remarkDirective);

const result = processor.processSync(`
> [!IMPORTANT]
> content
`);

console.log(result.toString());

// should output:
// :::info
// content
// :::