Add explicit definition for BlockString

[benjie]

As pointed out by @sungam3r in #899, we state:

In the type system definition language, these description strings (often {BlockString}) occur immediately before the definition they describe.

However, we don't have a definition for BlockString.

I've elevated BlockString to be its own entity, and StringValue now uses that definition instead.

GraphQL.js already has a BlockString token type: https://github.com/graphql/graphql-js/blob/b12dcffe83098922dcc6c0ec94eb6fc032bd9772/src/language/tokenKind.ts#L27 so I think this is just an edit to reflect reality and doesn't really require the RFC process.

@graphql/tsc Can I get some +1s? I'll also add it to the Sept 7 WG in case anyone feels we shouldn't do this.

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[fotoetienne]

Makes sense

[benjie]

With all these approvals I'm going to go ahead and merge this; we can always undo it if we need to 👍

Thanks everyone!