Add support for strict Liquid parsing

[charlespwd]

In this PR

• Add support for Strict Liquid Markup parsing

What this means

Right now, the LiquidHTML parser has default "fall through" cases for liquidTagOpen and liquidTag.

That is, we used to do something like this:

liquidTag = 
  | liquidTagAssign
  | liquidTagEcho
  | ...
  | liquidTagBaseCase

i.e., try the strict rule and, if that fails, parse the .markup as a string.

> toLiquidHTMLAST(`{% echo var %}`, { mode: 'tolerant' }) 
[
  {
    type: 'LiquidTag',
    name: 'echo',
    markup: {
      type: 'VariableLookup',
      expression: {/* ... /*}
      lookups: [],
    },
  }
]

> toLiquidHTMLAST(`{% echo var } error! { %}`, { mode: 'tolerant' }) 
[
  {
    type: 'LiquidTag',
    name: 'echo',
    markup: 'var } error! {', // this is a string
  }
]

With { mode: 'strict' }, we'll throw errors instead of accepting that kind of crappy Liquid.

> toLiquidHTMLAST(`{% echo var } error! { %}`, { mode: 'strict' }) 
LiquidHTMLParsingError: 
  Expected "%}", "-%}", "}}", "-}}", a space, "|", ".", or "["

Consequences

Nothing in prettier-plugin-liquid; but, in theme-check-js, this should be enough to power the LiquidSyntaxError check.