`{*}`: Add code comments to the `Components` if not present.

[chris-ramon]

Actual

Currently some of the Components do not have code comments.

Expected

We want to add code comments for better maintainability to the following Components if not present:

- Errors
- Language
  - Source
  - AST
  - Kinds
  - Lexer
  - Location
  - Parser
  - Printer
  - Visitor
- Executor
  - Resolver
- Validator
- Definition
- Directives
- Introspection
- Rules
- Schema

Reference:

• README: adds Components list chris-ramon/thesis-graphql-go#19
• https://github.com/chris-ramon/thesis-graphql-go/blob/1cda7f28af901d94d8d77fc0b2050493ed85aa9d/README.md#components

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