Support dictionary indexers on nested properties in F# lexer (#1482)

A dictionary indexer on a nested property currently breaks the F#
lexer. This commit adds support for these indexers by treating the `[`
as a character that triggers the `:dotted` state to be popped.

lib/rouge/lexers/fsharp.rb

@@ -113,6 +113,7 @@ def self.primitives
         rule %r/#{upper_id}(?=\s*[.])/, Name::Namespace
         rule upper_id, Name::Class, :pop!
         rule id, Name, :pop!
+        rule %r/\[/, Punctuation, :pop!
       end
     end
   end

spec/visual/samples/fsharp

@@ -198,3 +198,9 @@ finally
     traceEndBuild()
     if !TargetHelper.ExitCode.exitCode <> 0 then exit !TargetHelper.ExitCode.exitCode
     if Environment.ExitCode <> 0 then exit Environment.ExitCode
+
+let value = array.[0]
+let value = dict.Test.["key"]
+
+// syntax highlighting
+let number = 123