LONDON —Former Scottish First Minister Alex Salmond launched a new pro-independence party on Friday to run in May’s parliamentary election, a move that could splinter the separatist vote by grabbing support away from Nicola Sturgeon, his protege turned foe. British Prime Minister Boris Johnson has repeatedly rejected calls for a fresh vote on Scottish independence following the last one in 2014 that resulted in a vote to stay part of the United Kingdom. Opinion polls in the last 12 months have shown historically high support for independence. The May 6 election is seen as a critical test of public sentiment in Scotland, and an opportunity for the Scottish National Party that Sturgeon leads to put fresh pressure on Johnson. “I’m announcing the public launch of a new political force: the Alba Party,” Salmond said in a statement broadcast on Twitter. “Alba will contest the upcoming Scottish elections as a list-only …