Update 'variable' definition for on Liquid for Designers wiki page

I'm implementing a parser for Liquid and it seems like the 'Liquid for Designers' page has the closest thing to a robust spec of the language: https://github.com/Shopify/liquid/wiki/Liquid-for-Designers#expressions-and-variables

(I thought the website would be, but it doesn't define the format of variables: https://shopify.github.io/liquid/tags/variable)

It says that the only special character that can be in variables is an underscore _:

Variables. The most basic kind of expression is just the name of a variable. Liquid variables are named like Ruby variables: they should consist of alphanumeric characters and underscores, should always start with a letter, and do not have any kind of leading sigil (that is, they look like var_name, not $var_name).

However, the implementation also supports hyphens - in variable names:

> Liquid::Template.parse(" {{ hello-world }} ").render('hello-world' => "HELLO, WORLD!!!")
=> " HELLO, WORLD!!! "

AND

It also supports a question mark ? at the end of variable name
It looks like dashes have been supported in variables names since 2014: #478

> Liquid::Template.parse(" {{ hello-world? }} ").render('hello-world?' => "HELLO.... world?!?")
=> " HELLO.... world?!? "

(Both with Liquid::Template.error_mode = :strict declared beforehand)

The wiki is closed to outside contributors, or I'd edit it myself. I propose something like:

Variables. The most basic kind of expression is just the name of a variable. Liquid variables:

• consist of alphanumeric characters, underscores, and/or hypens AND
• always start with a letter, AND
• never start with any kind of leading sigil (that is, they look like var_name, not $var_name)
• may end with a single question mark

Read the Liquid for Designers wiki page at the linked Expressions and Variables section, and compare its variable rules with the examples in this issue and issue #478. Update the definition to include hyphens and a trailing question mark while retaining the starting-letter and no-leading-sigil rules; done when the page matches supported syntax.