slugs for foreign keys

[interim17]

Problem

Advances #27 and #7 Closes #18

Solution

Ongoing work on schema typing, resolvers, and relationships between entities in gatsby's data layer.

The focus of this pr is making this work:
dataset: MarkdownRemark @link(by: "fields.slug")

The idea is to have a foreign key relationship where one markdown file references another by its slug.

This takes a little bit more work than using the raw string from frontmatter.name but it makes the relationships less brittle. Because the slugs are /directory/filename they are far less likely to run into namespace collisions than a field line name.

Attendant to this I added:

• slugify as a dependency to help with slugs
• resolver utils for slugs and software entities
• vitest and one test suit for the resolver utils

• New feature (non-breaking change which adds functionality)