[Feature Request] Boost.regex - allow OR operator in look-behind

### Description

As the title says.

The lib doesnt accept OR operator in lookbehind.
Example:
```js
/\w+(?<=Sublime|Bob)/
```
doesnt work.

BUT it accepts this:
```js
/\w+(?<=Sublime)(?<=Bob)/
```

I would like the lib to support the OR operation for the regex look-behind
### Other Details
These don't work too:
```js
/\w+(?<=(?:Sublime|Bob))/
```
```js
/\w+(?<=\s?Sublime)/
```

The issue names no source files, tests, or entry points. Start by reproducing the listed look-behind examples in the existing Boost.Regex test setup, then trace how look-behind alternatives are parsed and validated; done means the OR, noncapturing-group, and optional-space examples work without regressing existing look-behind behavior.

Written by the indexing model from the issue text.