Add content_for tag parser support + ValidContentForArguments check

[charlespwd]

Fixes #466

What are you adding in this PR?

• Add parser support for the content_for tag
• Updated the old content_for checks to use the AST rather than regexes to perform their work
• Add prettier plugin support for the content_for tag
• Add the ValidContentForArguments check
  • For {% content_for "blocks" %}
    • Only context.* arguments are accepted
  • For {% content_for "block" %}
    • type and id are required
    • type and id must be strings
    • context.* arguments are optional
    • nothing else is accepted

What's next? Any followup issues?

• Document ValidContentForArguments on shopify.dev
• Support content_for "" first argument completion #569
• Support content_for "block" named argument completion #570
• Support content_for "block" type completion #571

What did you learn?

There were more things I needed to change than I remembered. TypeScript helps a ton here because it complains any time you add a new node type.

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • It's in the allChecks array in src/checks/index.ts
  • I ran yarn build and committed the updated configuration files
• I included a minor bump changeset
• My feature is backward compatible

[charlespwd]

Tagging @albchu and @miazbikowski for context sharing.

[charlespwd]

Prettier logic is dense.

The gist is that line is a space when the doc doesn't break on a new line, and a new line + indentation adjustment when the doc breaks.

What I'm writing here is that ContentForMarkup nodes should be printed as follows

// no arguments?
{% content_for 'contentForType' %}

// arguments & no break ?
{% content_for 'contentForType', arg1: value1, arg2: value2 %}

// arguments + break ?
{% content_for 'contentForType',
  arg1: value1,
  arg2: value2
%}

[charlespwd]

The tag(trailingWhitespace) helper is here to handle the logic about what "whitespace-logic-char" we want to use at the end of the tag.

// when it doesn't break
tag('') => {% content_for "blocks"%}
tag(' ') => {% content_for "blocks" %}
tag(line) => {% content_for "blocks" %}

// when it breaks
tag('') => {% content_for "blocks"%}
tag(' ') => {% content_for "blocks" %}
tag(line) => {% content_for "blocks"
%}

[aswamy]

small stuff