`''\x` not highlighted correctly in intended string

[andersk]

In Nix ''\ may be followed by any character, not just n/r/t:

Linefeed, carriage-return and tab characters can be written as ''\n, ''\r, ''\t, and ''\ escapes any other character.

One use case is described at NixOS/nix#1416 (comment).

However, nix-mode only special-cases ''\n, ''\r, ''\t, and incorrectly highlights ''\x as if it ends the string.

(Related: #81, fixed by #157.)