Adds ByteTables, moves cursor load order, consolidates byte constants

Adds ByteTables (lib/liquid/byte_tables.rb): four frozen 256-entry boolean
lookup arrays replacing inline byte-range comparisons throughout:
  IDENT_START, IDENT_CONT, DIGIT, WHITESPACE

Moves require 'liquid/cursor' to immediately after byte_tables in liquid.rb.
Cursor has zero Liquid dependencies; loading it early lets every subsequent
file reference Cursor:: constants directly.

Removes all local byte-constant definitions that duplicated Cursor::
  tokenizer.rb   OPEN_CURLEY / CLOSE_CURLEY / PERCENTAGE
  block_body.rb  OPEN_CURLEY_BYTE / PERCENT_BYTE / DASH_BYTE / CLOSE_CURLEY_BYTE
  expression.rb  DOT / DASH / ZERO / NINE / INTEGER_REGEX / FLOAT_REGEX

Replaces inline byte-range comparisons with ByteTables lookups in:
  cursor.rb, variable_lookup.rb, expression.rb, standardfilters.rb

Removes incidental dead code alongside the constant consolidation:
  tokenizer.rb: require 'strscan', unused string_scanner: param,
                @ss = nil, .to_s.to_str, 'tokenize if @source' guard
  block_body.rb: require 'English'

lib/liquid.rb

@@ -52,6 +52,8 @@ end
 require "liquid/version"
 require "liquid/deprecations"
 require "liquid/const"
+require 'liquid/byte_tables'
+require 'liquid/cursor'
 require 'liquid/standardfilters'
 require 'liquid/file_system'
 require 'liquid/parser_switching'
@@ -83,7 +85,6 @@ require 'liquid/expression'
 require 'liquid/template'
 require 'liquid/condition'
 require 'liquid/utils'
-require 'liquid/cursor'
 require 'liquid/tokenizer'
 require 'liquid/parse_context'
 require 'liquid/partial_cache'