Adding regexp for identifier names

[bendbennett]

This PR adds a JSON schema definition for a regular expression that conforms to the requirements for a Terraform configuration language identifier.

    "valid_identifier": {
      "type": "string",
      "pattern": "^[a-z_][a-z0-9_]*$"
    }

This validation has been added to all data source, provider and resource names, and all attribute and block names.

For example:

    "datasource": {
      "type": "object",
      "properties": {
        "name": {
          "type": "string",
          "$ref": "#/$defs/valid_identifier"
        },
        "schema": {
          "type": "object",
          "$ref": "#/$defs/datasource_schema"
        }
      },
      "required": [
        "name",
        "schema"
      ]
    },

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