Explicitly define mutable strings

Ruby 2.3 introduced a magic comment, `frozen_string_literal: true`, that
allows strings to be immutable by default. Ruby 3.4 is preparing to make
this the default behavior and throws deprecation warnings when mutating
strings.

There are many ways to make a mutable string, but I've chosen to use
`String.new` because it is the most explicit option and also doesn't
require creating multiple string objects

lib/roo/formatters/yaml.rb

@@ -13,7 +13,7 @@ def to_yaml(prefix = {}, from_row = nil, from_column = nil, to_row = nil, to_col
         from_column ||= first_column(sheet)
         to_column ||= last_column(sheet)
 
-        result = "--- \n"
+        result = String.new("--- \n")
         from_row.upto(to_row) do |row|
           from_column.upto(to_column) do |col|
             next if empty?(row, col, sheet)

test/roo/test_open_office.rb

@@ -224,7 +224,7 @@ def test_bug_ric
       assert workbook.empty?("C", 1)
       assert workbook.empty?("D", 1)
       expected = 1
-      letter = "e"
+      letter = String.new("e")
       while letter <= "u"
         assert_equal expected, workbook.cell(letter, 1)
         letter.succ!