Lexer: show helpful error message against invalid number literal like…

… '.42'

Dot-started floating-point number like '.42' is not supported on
Crystal. However compiler does not tell us this, so this fix adds to
detect such a literal and to show helpful error message.

spec/compiler/lexer/lexer_spec.cr

@@ -321,6 +321,9 @@ describe "Lexer" do
   assert_syntax_error "4F64", "unexpected token: F64"
   assert_syntax_error "0F32", "unexpected token: F32"
 
+  assert_syntax_error ".42", ".1 style number literal is not supported, put 0 before dot"
+  assert_syntax_error "-.42", ".1 style number literal is not supported, put 0 before dot"
+
   it "lexes not instance var" do
     lexer = Lexer.new "!@foo"
     token = lexer.next_token

src/compiler/crystal/syntax/lexer.cr

@@ -636,6 +636,8 @@ module Crystal
       when '~'
         next_char :"~"
       when '.'
+        line = @line_number
+        column = @column_number
         case next_char
         when '.'
           case next_char
@@ -644,6 +646,8 @@ module Crystal
           else
             @token.type = :".."
           end
+        when .ascii_number?
+          raise ".1 style number literal is not supported, put 0 before dot", line, column
         else
           @token.type = :"."
         end