Add more test cases for prefer-string-raw rule

[som-sm]

This PR adds a couple of new test cases that cover the following:

• String.raw`a = '\\'`

  This covers the following check, which is currently not covered by any of the tests.
  

  eslint-plugin-unicorn/rules/prefer-string-raw.js

  Line 59 in 944cb50

  raw.at(-2) === BACKSLASH

•  String.raw`a = 'a\\b\''`
   String.raw`a = "a\\b\""`

  These cover the nextCharacter === quote check in the following line, which again is not currently covered.
  

  eslint-plugin-unicorn/rules/prefer-string-raw.js

  Line 21 in 944cb50

  if (nextCharacter === BACKSLASH || nextCharacter === quote) {

•  String.raw`a = 'a\\b\"'`

  Finally, this just highlights that if the string contains unnecessarily escaped quotes (or unnecessary escapes in general), this rule would stop complaining.

[som-sm]

Also, would you be open for the following refactor?

Simplifying the following:

eslint-plugin-unicorn/rules/prefer-string-raw.js

Lines 11 to 32 in 944cb50

	function unescapeBackslash(raw) {
	const quote = raw.charAt(0);
	raw = raw.slice(1, -1);
	let result = '';
	for (let position = 0; position < raw.length; position++) {
	const character = raw[position];
	if (character === BACKSLASH) {
	const nextCharacter = raw[position + 1];
	if (nextCharacter === BACKSLASH || nextCharacter === quote) {
	result += nextCharacter;
	position++;
	continue;
	}
	}
	result += character;
	}
	return result;
	}

To this:

function unescapeBackslash(raw) {
	const quote = raw.charAt(0);

	return raw
		.slice(1, -1)
		.replaceAll(new RegExp(String.raw`\\([\\${quote}])`, 'g'), '$1');
}

It uses a regex to replace Backslash + Backslash with Backslash and Backslash + quote with quote.

[fisker]

would you be open for the following refactor?

Seems safe to apply.

But please use a named group instead.

[som-sm]

@fisker Sure, done in #2692.