Support %=...= percent string literals

This is special-cased so it doesn't match the modulo assignment
operator.

Author: dkearns

spec/syntax/operators_spec.rb

@@ -8,6 +8,12 @@
     vim.command 'unlet g:ruby_operators'
   end
 
+  specify "modulo-assignment operators" do
+    assert_correct_highlighting(<<~'EOF', '%=', 'rubyAssignmentOperator')
+      foo %= bar
+    EOF
+  end
+
   specify "ternary operators" do
     str = <<~'EOF'
       foo = bar ? 4 : 2

spec/syntax/strings/strings_spec.rb

@@ -0,0 +1,9 @@
+require 'spec_helper'
+
+describe "Syntax highlighting" do
+  specify "percent strings with a modulo-assignment operator look-alike delimiter" do
+    assert_correct_highlighting(<<~'EOF', '%=', 'rubyPercentStringDelimiter')
+      foo = %= bar =
+    EOF
+  end
+end

syntax/ruby.vim

@@ -290,9 +290,9 @@ SynFold '%' syn region rubySymbol matchgroup=rubyPercentSymbolDelimiter start="%
 SynFold '%' syn region rubySymbol matchgroup=rubyPercentSymbolDelimiter start="%i("	  end=")"   skip="\\\\\|\\)"   contains=rubyBackslashEscape,rubySpaceEscape,rubyParenthesisEscape,rubyNestedParentheses
 
 " Generalized Double Quoted Strings, Array of Strings, Array of Symbols and Shell Command Output {{{1
-" Note: %= is not matched here as the beginning of a double quoted string
+SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="\%(\%(\w\|[^\x00-\x7F]\|]\)\s*\)\@<!%=" end="="	 skip="\\\\\|\\="   contains=@rubyStringSpecial nextgroup=@rubyModifier skipwhite
 SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%\z([~`!@#$%^&*_\-+|\:;"',.?/]\)"       end="\z1" skip="\\\\\|\\\z1" contains=@rubyStringSpecial nextgroup=@rubyModifier skipwhite
-SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%[QWx]\z([~`!@#$%^&*_\-+=|\:;"',.?/]\)" end="\z1" skip="\\\\\|\\\z1" contains=@rubyStringSpecial
+SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%[QWx]\z([~`!@#$%^&*_\-+=|\:;"',.?/]\)" end="\z1" skip="\\\\\|\\\z1" contains=@rubyStringSpecial nextgroup=@rubyModifier skipwhite
 SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%[QWx]\={"			       end="}"	 skip="\\\\\|\\}"   contains=@rubyStringSpecial,rubyNestedCurlyBraces
 SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%[QWx]\=<"			       end=">"	 skip="\\\\\|\\>"   contains=@rubyStringSpecial,rubyNestedAngleBrackets
 SynFold '%' syn region rubyString matchgroup=rubyPercentStringDelimiter start="%[QWx]\=\["			       end="\]"  skip="\\\\\|\\\]"  contains=@rubyStringSpecial,rubyNestedSquareBrackets