[WIP] Support adding directives as docblock tags

[captbaritone]

This is a diff exploring adding the ability to attach directives to different constructs. I works by interpreting docblock tags (which look like directives!) as directives.

A tag is interpreted as a directive if:

1. It has no value, and is a defined directive in the schema
2. It is written with arguments: @someDirective(someArg: "foo")

This is very light weight and intuitive (I think) since it does not require any novel tag name or syntax for attaching directives. However, it comes at the cost of some ambiguity. This could be especially problematic if there is a namespace collision between TypeScript tag names and directive names in your schema.

Note that this PR does not (yet?) include the ability to define directives, so it hasn't yet been tested end to end.

Todo

• Add ability to define directives
• Add tests for each position where we expect/accept directives

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