Allow testing string sources

[tommy-mitchell]

Closes #86.

Allows testing files passed to tsd as strings, either programmatically:

import tsd from 'tsd';

const diagnostics = await tsd({
	testFiles: [
		'file1.ts',
		{
			'name': 'file2.ts',
			'text': 'import {expectType} from \'tsd\';\nexpectType<number>(1);',
		},
	],
});

or via the CLI with a concise syntax (file.extension:source):

$ npx tsd --files "foo.ts:$(cat output.txt)"

Type definitions have been updated like so:

type TestFiles = ReadonlyArray<(
	| string
	| {name: string; text: string}
)>;

type SourceFiles = ReadonlyArray<{name: string; text: string}>;

type ParsedTestFiles = {
	globs: readonly string[];
	sourceFiles?: SourceFiles;
};

interface Context {
	cwd: string;
	pkg: PackageJsonWithTsdConfig;
	typingsFile: string;
	testFiles: ParsedTestFiles;
	config: Config;
}

interface Options {
	cwd: string;
	typingsFile?: string;
	testFiles?: TestFiles;
}

---

Still some items to finish, but opening this PR to get feedback:

• Document feature in readme and CLI help
• Add documentation comments to added functions/types
• Improve names/concepts (e.g. globs vs. sourceFiles, etc.)
• Consolidate repetition of some types
• Make sure that parsing via the CLI is robust
• See if input needs to be escaped

[sindresorhus]

Just a warning. I have very little time to spend on this project. So I will not be able to review non-essential things. I'm just helping out to keep it up to date with TS releases. So if you plan to make a lot of changes, I would recommend reaching out to @SamVerschueren to become a maintainer.