Add @Custom:* NatSpec tag highlighting

Solidity supports @Custom: NatSpec tags (e.g. @Custom:oz-upgrades,
@Custom:smtchecker) since v0.6.2. This adds them to the existing
solNatSpec match pattern so they highlight consistently with @title,
@author, @notice, etc.

Author: evertonfraga

syntax/solidity.vim

@@ -165,7 +165,7 @@ hi def link solEventArgSpecial   Label
 
 " Comment
 syn keyword solCommentTodo       TODO FIXME XXX TBD contained
-syn match solNatSpec             contained /@title\|@author\|@notice\|@dev\|@param\|@inheritdoc\|@return/
+syn match solNatSpec             contained /@title\|@author\|@notice\|@dev\|@param\|@inheritdoc\|@return\|@custom:\w\+/
 syn region  solLineComment       start=+\/\/+ end=+$+ contains=solCommentTodo,solNatSpec,@Spell
 syn region  solLineComment       start=+^\s*\/\/+ skip=+\n\s*\/\/+ end=+$+ contains=solCommentTodo,solNatSpec,@Spell fold
 syn region  solComment           start="/\*"  end="\*/" contains=solCommentTodo,solNatSpec,@Spell fold