Use String#match? instead of String#=~ to reduce allocation for backreferecne

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

WhitespaceOrNothing = /\A\s*\z/
token = " " * 20
token =~ WhitespaceOrNothing

Benchmark.ips do |x|
  x.report("=~") {
    token =~ WhitespaceOrNothing
  }
  x.report("match?") {
    token.match?(WhitespaceOrNothing)
  }

  x.compare!
end
```

## Result
```
Warming up --------------------------------------
                  =~   271.356k i/100ms
              match?   579.655k i/100ms
Calculating -------------------------------------
                  =~      2.717M (± 0.4%) i/s -     13.839M in   5.092947s
              match?      5.695M (± 1.6%) i/s -     28.983M in   5.090640s

Comparison:
              match?:  5694747.3 i/s
                  =~:  2717370.9 i/s - 2.10x  (± 0.00) slower
```

lib/liquid/block_body.rb

@@ -37,7 +37,7 @@ module Liquid
 
     private def parse_for_liquid_tag(tokenizer, parse_context)
       while (token = tokenizer.shift)
-        unless token.empty? || token =~ WhitespaceOrNothing
+        unless token.empty? || token.match?(WhitespaceOrNothing)
           unless token =~ LiquidTagToken
             # line isn't empty but didn't match tag syntax, yield and let the
             # caller raise a syntax error
@@ -150,7 +150,7 @@ module Liquid
           end
           parse_context.trim_whitespace = false
           @nodelist << token
-          @blank &&= !!(token =~ WhitespaceOrNothing)
+          @blank &&= token.match?(WhitespaceOrNothing)
         end
         parse_context.line_number = tokenizer.line_number
       end