add rule to detect useless escapes

[ajafff]

In the git history of trailing-newline.ts you can find a mistake where I wrote '\UFEFF' instead of \uFEFF. The rule should check every string and template literal for escaped characters that are useless. Some background on escape sequences: https://mathiasbynens.be/notes/javascript-escapes

It doesn't need to check for useless escapes in regular expressions.

It should allow '\${' and `\${` only if followed by { to allow for escaping template literal substitutions (or suppress TSLint's no-invalid-template-strings).

Note that this information is not present in the AST node, so you need to use node.getText() to get the raw source text.

The autofixer could just remove the backslash.

[ajafff]

The rule should probably exclude tagged template literals, because the tag function can actually access the content of the original escape and may use that for some strange magic. https://tc39.github.io/proposal-template-literal-revision/

[ajafff]

Octal escapes in string literal types are not parsed, the escape is therefore useless. But that may also just be a bug: https://github.com/Microsoft/TypeScript/issues/24209