Support custom markup delimiters

[jg-rp]

This PR refactors Liquid2::Scanner and changes Liquid2::Environment to allow for custom markup delimiters, which default to {{ and }} for output statements, {% and %} for tags and {# and #} for comments.

For example, we can change all those curly brackets to be square brackets.

require "liquid2"

env = Liquid2::Environment.new(
  markup_out_start: "[[",
  markup_out_end: "]]",
  markup_tag_start: "[%",
  markup_tag_end: "%]",
  markup_comment_prefix: "[#",
  markup_comment_suffix: "]"
)

source = "[% if true %]Hello, [[ you ]]![% endif %]"
puts env.render(source, "you" => "World")

And configure multiple environments with different delimiters.

env = Liquid2::Environment.new(
  markup_out_start: "[[",
  markup_out_end: "]]"
)

source = "Hello, [[ you ]]!"

another_env = Liquid2::Environment.new(
  markup_out_start: "((",
  markup_out_end: "))"
)

another_source = "Hello, (( you ))!"

puts env.render(source, "you" => "World")
puts another_env.render(another_source, "you" => "Liquid")

This feature does mean the tokenizer is slightly slower now then when we had hard coded delimiters, but I'm inclined to go with it.

A word of warning though, both ]] and )) from the examples above can appear in standard Liquid expressions. So one would need to pick more elaborate delimiters that don't include existing punctuation, or accept that some syntax will not be available.