Use String#=~ and Regexp.last_match instead to retrieve the markup content

If the first value is only used obtained with String#scan,
it will increase the performance if replace with `String#=~` and `Regexp.last_match`.

### Environment
- MacBook Air (M1, 2020)
- macOS 12.0 beta 7
- Apple M1
- Ruby 3.0.2

### Test code
```ruby
require 'benchmark/ips'

WhitespaceControl           = '-'
VariableStart               = /\{\{/
VariableEnd                 = /\}\}/

ContentOfVariable   = /\A#{VariableStart}#{WhitespaceControl}?(.*?)#{WhitespaceControl}?#{VariableEnd}\z/om
token = "{{item.product.featured_image | product_img_url: 'thumb' }}"

Benchmark.ips do |x|
  x.report("String#scan")  { token.scan(ContentOfVariable) {|content| break }  }
  x.report("String#match") { m = token.match(ContentOfVariable); m[1] }
  x.report("String#=~")    { token =~ ContentOfVariable; Regexp.last_match(1) }

  x.compare!
end
```

### Result
```
Warming up --------------------------------------
         String#scan   135.724k i/100ms
        String#match   117.397k i/100ms
           String#=~   151.637k i/100ms
Calculating -------------------------------------
         String#scan      1.351M (± 0.8%) i/s -      6.786M in   5.021955s
        String#match      1.169M (± 1.3%) i/s -      5.870M in   5.020429s
           String#=~      1.520M (± 0.9%) i/s -      7.733M in   5.087427s

Comparison:
           String#=~:  1520250.9 i/s
         String#scan:  1351399.0 i/s - 1.12x  (± 0.00) slower
        String#match:  1169384.1 i/s - 1.30x  (± 0.00) slower
```

lib/liquid/block_body.rb

@@ -231,8 +231,8 @@ module Liquid
     end
 
     def create_variable(token, parse_context)
-      token.scan(ContentOfVariable) do |content|
-        markup = content.first
+      if token =~ ContentOfVariable
+        markup = Regexp.last_match(1)
         return Variable.new(markup, parse_context)
       end
       BlockBody.raise_missing_variable_terminator(token, parse_context)