Remove backslash line continuations from the CSS output

[wahidrizka]

The README lists "Backslash before new line" among the 3 ways to continue a line for any type of node. The parser keeps the backslash in selectors, at-rule params and declaration values, so the CSS output still has it:

@media screen and \
       (min-width: 600px) {

A backslash before a new line is not a valid escape in CSS, and browsers reject it. In Chrome 153 the media query above becomes not all, an @supports rule written the same way is dropped, and so is a margin value continued with a backslash. With only the backslash removed, all three apply.

Now the parser removes the backslash from the value and from raws[prop].raw, and keeps it in raws[prop].sss, like it does for inline comments, so stringify() still returns the original SugarSS. The new backslash case fails on main; it covers at-rule params, a selector and a declaration value. All 66 tests pass.

Related to #91: with this, a multi-line @apply px-10 \ works with Tailwind CSS 4.3.3, while with sugarss 5.0.1 Tailwind reads the backslash as a class name. When the classes start on the next line (@apply \), params still starts with a new line, which Tailwind reads as an empty class. Fixing that needs the new line in raws.afterName and a SugarSS-only raw to keep the round trip, so I left it out; I can add it if you want.

I made backslash.json with JSON.stringify(jsonify(root), null, 2), because test/update-cases.js now writes [object Object]: jsonify() from postcss-parser-tests 8.9.0 returns an object.

[ai]

Thanks. Released in 5.0.2.