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

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