[LESS] [SCSS/SASS] Add support for single‑line comments

**LESS** and **SCSS**/**SASS**, unlike **CSS**, all allow single‑line comments, but **Pygments** fails to highlight those as a comment at the root level of **LESS**, and content level of **SCSS**/**SASS**:

<dl>
<dt>LESS</dt>
<dd>

```less
// single-line comment
```
</dd>
<dt>SCSS</dt>
<dd>

```scss
// single-line comment
```
</dd>
<dt>SASS</dt>
<dd>

```sass
// single-line comment
```
</dd>
</dl>