Support options in external plugins

[azz]

#3433 moved the definition of options to our internal parsers but there's still a bit of work to support external plugins defining new options:

Use cases:

• This should include the options contributed by plugins:

  prettier --plugin @prettier/plugin-python --help

• CLI should work:

  prettier --plugin my-cool-prettier-plugin --my-custom-option

• Remove unknown option warning for options contributed by external plugins.

  prettier.format('def foo(): pass', {
    parser: 'python',
    plugins: ['@prettier/plugin-python'],
    pythonVersion: '3.1.4'
  })
  ● Validation Warning:
  
   Unknown option "pythonVersion" with value "3.1.4" was found.
   This is probably a typing mistake. Fixing it will remove this message.

[taion]

I don't believe #3433 did that. I believe it just exposed (via some code duplication) plugin options in getSupportInfo().

At the moment, all options other than the ones in cli/constant get dropped:

prettier/src/cli/util.js

Lines 628 to 642 in 6c0dd74

	let option = constant.detailedOptionMap[key];
	if (type === "api" && option === undefined) {
	option = constant.apiDetailedOptionMap[key];
	}
	// unknown option
	if (option === undefined) {
	if (type === "api") {
	consoleWarn(`Ignored unknown option: ${rawKey}`);
	} else {
	const optionName = rawValue === false ? `no-${rawKey}` : rawKey;
	consoleWarn(`Ignored unknown option: --${optionName}`);
	}
	return;
	}

In fact this happens before plugins are even loaded.

Unless I'm missing something, supporting per-plugin options would require a revamp of option parsing here. You'd need to first load plugins, then load their options, then parse the rest of the config.

[azz]

It will just need to key into main/options.js's normalize. Shouldn't be too much work.

[taion]

Yeah, something like that. There are a few different code paths here depending on which entry point you use. Also a bit harder for CLI stuff if relevant. Nothing too bad though.

Though it is worth thinking about how to configure e.g. trailing commas for Python. Something other than --trailing-comma would be most straightforward, but slightly messy.

[duailibe]

@ikatyang has already planned a follow-up PR to deduplicate work with normalize. #3433 was just the start in that direction to unblock some other stuff (like #3573)

[ikatyang]

use supportOptions to generate CLI options (internal plugins) --> #3622