Feature summary (what you would like to be able to do and where):
Add args parameter for parse action of action API to pass wikitext source or page to be parsed.

For example, when I request these params to API:

{
	"action": "parse",
	"format": "json",
	"title": "Hello",
	"text": "Hello, {{{1}}}!",
	"args": "1=World",
	"formatversion": 2
}

the current parse result will be:

<div class="mw-parser-output"><p>Hello, {{{1}}}!</p></div>

but I want to get

<div class="mw-parser-output"><p>Hello, World!</p></div>

I think the format of args parameter would be good to be similar to change param in options action.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

• Simulating arguments for the template on its edit page
• Wikitext-based internationalization or customization for gadgets or user scripts
• Integrating wikitext templates on Vue app
• Lazy rendered wikitext contents generated by client-side scripts

Benefits (why should this be implemented?):
We can use wikitext as a template engine for client-side scripts.