Add validation schema for configuration file

[fruzitent]

Things to consider:

• Generate schema from Python dataclass definition using pydantic or similar package.

[BlankSpruce]

I'll take a look at this PR on the weekend. Could you give me a hint how to use such schema? What does it solve?

[fruzitent]

I'll take a look at this PR on the weekend. Could you give me a hint how to use such schema? What does it solve?

• IDE integration;
• Validation of properties in user config file.

Header on the first line, tells LSP which schema to use.

• json

  { "$schema": "https://" }

• toml

  #:schema https://

• yml

  # yaml-language-server: $schema=https://

Quite a few examples of these can be found at https://schemastore.org.

[BlankSpruce]

Although I don't use this tooling myself and I don't expect to change .gersemirc per project too often I rather like the idea of making configuration more user-friendly. I've reimplemented most of this change (description below) but I've still made you the main author of this change.

Significant changes to your original version:

• I used metadata achievable with dataclasses.field and I implemented something similar for Enum
• As per your recommendation I used pydantic to implement the test that will warn me about deviation to schema and example file whenever I touch something around there. While it wasn't very helpful with enumeration (it wasn't as detailed as your original schema) it made a difference for definitions argument (format part):

    "definitions": {
      "default": [],
      "description": "Files or directories containing custom command definitions (functions or macros). If only - is provided custom definitions, if there are any, are taken from stdin instead. Commands from not deprecated CMake native modules don't have to be provided. See: https://cmake.org/cmake/help/latest/manual/cmake-modules.7.html",
      "items": { "format": "path", "type": "string" },
      "title": "Definitions",
      "type": "array"
    },

• I added yaml header to default configuration produced by --default-config.
• I squashed your changes into one commit. I'm glad though you took the approach of splitting the work into meaningful commits. :)

Once I solve the issue #13 I hope to release new version with your changes. Thank you for your contribution.