Update devDependencies and add a named export

[ecraig12345]

• Add a named export for the pGraph function
• Use import type/export type where relevant
• Update devDependencies to latest versions
• Update local Node version to 20. This should have no impact on consumption of the package, since it doesn't use any Node APIs and the TS options should help prevent usage of newer ES builtins.
• Add Prettier formatting and reduce printWidth to 100
• For tests, move expect extensions and test fixtures to a separate file