Support for indent-blankline v3

[UtkarshVerma]

The project was refactored at v3 and now the highlight groups have changed, which are covered in the Migration Guide.

Here's a snippet from the help page:

==============================================================================
 4. HIGHLIGHTS                                                *ibl.highlights*

IblIndent                                                       *hl-IblIndent*

 The default highlight group for indentation characters.

 Default: takes the values from |hl-Whitespace| when not defined ~

IblWhitespace                                               *hl-IblWhitespace*

 The default highlight group for whitespace characters.

 Default: takes the values from |hl-Whitespace| when not defined ~

IblScope                                                         *hl-IblScope*

 The default highlight group for |ibl.config.scope| characters.

 Default: takes the values from |hl-LineNr| when not defined ~

[loctvl842]

Thanks a lot, will do.

[UtkarshVerma]

Hi, did this get implemented? I still have to use the following overrides to get it to work:

[loctvl842]

can you create a PR, because I can't detect what are those highlight group represent for?