Drop base64 gem from dependency

This following warning is shown in test.
```
/Users/ryunosuke.sato/src/github.com/lautis/uglifier/spec/spec_helper.rb:3: warning: base64 was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.4.0.
You can add base64 to your Gemfile or gemspec to silence this warning.
```

Instead of adding base64 to dependency, the usage of `Base64#strict_encode64` and `Base64#strict_decode64` have replaced with `Array#pack` and `String#unpack1`.

lib/uglifier.rb

@@ -1,7 +1,6 @@
 # encoding: UTF-8
 
 require "json"
-require "base64"
 require "execjs"
 require "uglifier/version"
 
@@ -160,7 +159,7 @@ def initialize(options = {})
   def compile(source)
     if @options[:source_map]
       compiled, source_map = run_uglifyjs(source, true)
-      source_map_uri = Base64.strict_encode64(source_map)
+      source_map_uri = [source_map].pack('m0')
       source_map_mime = "application/json;charset=utf-8;base64"
       compiled + "\n//# sourceMappingURL=data:#{source_map_mime},#{source_map_uri}"
     else
@@ -512,7 +511,7 @@ def input_source_map(source, generate_map)
     source_map_options = @options[:source_map].is_a?(Hash) ? @options[:source_map] : {}
     sanitize_map_root(source_map_options.fetch(:input_source_map) do
       url = extract_source_mapping_url(source)
-      Base64.strict_decode64(url.split(",", 2)[-1]) if url && url.start_with?("data:")
+      url.split(",", 2)[-1].unpack1('m0') if url && url.start_with?("data:")
     end)
   rescue ArgumentError, JSON::ParserError
     nil