Support for php.vim plugin (#263)

This is a follow up for #218 [1] which added
highlighting groups for the bundled PHP syntax, but the groups are
actually defined by the StanAngeloff/php.vim [2] plugin.
Therefore the added highlighting calls will be moved to a plugin
section.
Additionally, the `phpClassExtends` and `phpClassImplements` groups
have been added to improve the highlighting for classes that implement
or extended interfaces/classes. The `phpUseClass` has also been added to
improve the highlighting for imports.

[1]: #218
[2]: https://github.com/StanAngeloff/php.vim

Related to GH-218
Closes GH-262

Co-authored-by: Sven Greb <development@svengreb.de>

colors/nord.vim

@@ -470,12 +470,9 @@ hi! link markdownHeadingDelimiter Keyword
 call s:hi("perlPackageDecl", s:nord7_gui, "", s:nord7_term, "", "", "")
 
 call s:hi("phpClasses", s:nord7_gui, "", s:nord7_term, "", "", "")
-call s:hi("phpClass", s:nord7_gui, "", s:nord7_term, "", "", "")
 call s:hi("phpDocTags", s:nord7_gui, "", s:nord7_term, "", "", "")
 hi! link phpDocCustomTags phpDocTags
 hi! link phpMemberSelector Keyword
-hi! link phpMethod Function
-hi! link phpFunction Function
 
 call s:hi("podCmdText", s:nord7_gui, "", s:nord7_term, "", "", "")
 call s:hi("podVerbatimLine", s:nord4_gui, "", "NONE", "", "", "")
@@ -755,6 +752,15 @@ hi! link mkdLinkDefTarget mkdURL
 hi! link mkdLinkTitle mkdInlineURL
 hi! link mkdDelimiter Keyword
 
+" PHP
+" > StanAngeloff/php.vim
+call s:hi("phpClass", s:nord7_gui, "", s:nord7_term, "", "", "")
+call s:hi("phpClassImplements", s:nord7_gui, "", s:nord7_term, "", s:bold, "")
+hi! link phpClassExtends phpClass
+hi! link phpFunction Function
+hi! link phpMethod Function
+hi! link phpUseClass phpClass
+
 " Vimwiki
 " > vimwiki/vimwiki
 if !exists("g:vimwiki_hl_headers") || g:vimwiki_hl_headers == 0