Add lang global directive and constructor option

[yhatt]

Marpit Markdown document becomes able to assign lang attribute for each slide through lang global directive or constructor option.

---
lang: en-US
---

...

or:

new Marpit({ lang: 'en-US' })

The constructor option is for setting the default lang attribute value, and Markdown author can override lang attribute by the global directive.

Output

<section lang="en-US">
  ...
</section>

Related issues in Marp ecosystem

• Add support for lang directive marp-vscode#430
• Add support for lang directive marp-cli#542