LSIF diagnostics GraphQL API proposal

Created by: felixfbecker

This is a GraphQL API to query LSIF diagnostics, which would be used by code insights and can also be displayed in the UI when browsing files (#4102).

Code insights will query diagnostics.pageInfo.totalCount for 7 past revisions on the current directory (which may be the root directory of the repo), for a specific tool (e.g. the ESLint extension would filter to eslint).

The blob component would query diagnostics on the current file with ranges, messages, source and severity.

References:

• https://microsoft.github.io/language-server-protocol/specifications/specification-current/#diagnostic
• https://github.com/microsoft/language-server-protocol/blob/master/indexFormat/specification.md#request-textdocumentdiagnostic

Differences:

• Used Location instead of Range to expose the file path
• Omitted relatedInformation for now for simplicity
• Made code always a string because GraphQL does not support scalar union types (backend can convert numbers to strings)
• Enums are GraphQL enums
• Made tags non-nullable, they are only optional in LSP for BC, backend can fill them with empty list if not provided

Future extensions of this would be:

• Querying diagnostic counts across all repositories
• Easy/performant way to query data at multiple past points in time at once
• Filter by severity etc.