Improve TOKENIZER by 23%

From what I can see, this is done in linear time: 4*O(n)
This tokenizer change converts that to something a little quicker: 3*O(n)

Seems that not using a capture group and something other than split would be a big win.
Other than that, the changes were meager.

I used https://regex101.com/ (and pcre2) to evaluate the cost of the TOKENIZER.
I verified with cruby 3.0.6

```
 /(%%\{[^\}]+\}|%\{[^\}]+\})/ =~ '%{{'*9999)+'}'

/(%%\{[^\}]+\}|%\{[^\}]+\})/ ==> 129,990 steps
/(%?%\{[^\}]+\})/            ==> 129,990 steps
/(%%?\{[^\}]+\})/            ==>  99,992 steps (simple savings of 25%) <===
/(%%?\{[^%}{]+\})/           ==>  89,993 steps (limiting variable contents has minimal gains)
```

Also of note are the null/simple cases:

```
/x/ =~ '%{{'*9999)+'}'
/x/                          ==>  29,998 steps
/(x)/                        ==>  59,996 steps
/%{x/                        ==>  49,998 steps
/(%%?{x)/                    ==>  89,993 steps
```

And comparing against a the plain string of the same length.

```
/x/ =~ 'abb'*9999+'c'

/x/                          ==>  29,999
/(%%?{x)/                    ==>  59,998
/(%%?\{[^\}]+\})/            ==>  59,998
/(%%\{[^\}]+\}|%\{[^\}]+\})/ ==>  89,997
```

per ruby-i18n#667

lib/i18n/backend/interpolation_compiler.rb

@@ -21,7 +21,7 @@ module InterpolationCompiler
       module Compiler
         extend self
 
-        TOKENIZER                    = /(%%\{[^\}]+\}|%\{[^\}]+\})/
+        TOKENIZER                    = /(%%?\{[^}]+\})/
         INTERPOLATION_SYNTAX_PATTERN = /(%)?(%\{([^\}]+)\})/
 
         def compile_if_an_interpolation(string)