fix(indent-blankline): update highlight groups for indent-blankline version 3

[dn9uyen]

Indent-blankline version 3 changes the highlight groups which means the indentation guides no longer highlight properly.

The following highlight groups have been renamed to match the new names:

• IndentBlanklineContextChar -> Iblscope
• IndentBlanklineChar -> IblIndent

The highlight groups IndentBlanklineSpaceChar and IndentBlanklineSpaceCharBlankline have been removed from Indent-blankline version 3 so the highlight links are no longer needed.

[antoineco]

Thanks for you contribution, much appreciated! 🙌

I am not expecting everybody to be on v3 already, so could you please keep the old highlight groups as a link to the new ones, to make the transition smoother?

call gruvbox_material#highlight('IblScope', s:palette.grey1, s:palette.none, 'nocombine')
call gruvbox_material#highlight('IblIndent', s:palette.bg5, s:palette.none, 'nocombine')
highlight! link IndentBlanklineContextChar IblScope
highlight! link IndentBlanklineChar IblIndent
highlight! link IndentBlanklineSpaceChar IndentBlanklineChar
highlight! link IndentBlanklineSpaceCharBlankline IndentBlanklineChar