fix: preserve consecutive backslashes in attribute values

[spokodev]

Problem

quoteAttribute serializes an attribute value with JSON.stringify (which doubles every backslash) and then tries to undo the doubling with .replace(/([^\\])\\/g, '$1'). That regex requires a non-backslash before each backslash and matches non-overlapping, so it cannot collapse a run of consecutive backslashes. The value gains an extra backslash on every serialization and no longer round-trips:

const { parse } = require('node-html-parser'); // 8.0.2

const el = parse('<div></div>').firstChild;
el.setAttribute('path', 'C:\\Users\\me');      // value: C:\Users\me  (single)  -> OK
el.setAttribute('path', 'C:\\\\Users\\\\me');  // value: C:\\Users\\me (doubled)
el.toString();                                 // <div path="C:\\\Users\\\me">  (extra backslash)
parse(el.toString()).firstChild.getAttribute('path'); // 'C:\\\Users\\\me'  (corrupted)

The single-backslash case fixed in #306 works; consecutive backslashes (e.g. a JSON-escaped Windows path or a regex stored in an attribute) were the unhandled residual.

Fix

Attribute values are literal text, so only the double quote needs escaping (as "). Quote the value directly instead of round-tripping it through JSON.stringify. This preserves any number of backslashes and still escapes embedded quotes (the #62 behaviour).

Test

Added a round-trip assertion for an attribute value with consecutive backslashes. It fails before the change and passes after; the full suite (263 passing) and lint are green.