Terraformで未宣言の変数がある場合のエラー対処

# terraform環境
```s
$ terraform -version
Terraform v1.6.1
on darwin_arm64
+ provider registry.terraform.io/hashicorp/aws v5.34.0

Your version of Terraform is out of date! The latest version
is 1.7.1. You can update by downloading from https://www.terraform.io/downloads.html
```

# network.tf
```terraform
# ----------------------------------
# Create a VPC
# ----------------------------------
resource "aws_vpc" "vpc" {
  cidr_block                       = "192.168.0.0/20"
  instance_tenancy                = "default"
  enable_dns_support               = true
  enable_dns_hostnames             = true
  assign_generated_ipv6_cidr_block = false

  tags = {
    Name    = "${var.project}-${var.environment}-vpc"
    Project = var.project
    Env     = var.environment
  }
}
```

# terraform.tfvars
```terraform
project     = "tastylog"
environment = "dev"
```

# terraform plan の結果
```s
╷
│ Warning: Value for undeclared variable
│ 
│ The root module does not declare a variable named "environment" but a value was found in file "terraform.tfvars". If you meant to use this value, add a "variable" block to the configuration.
│ 
│ To silence these warnings, use TF_VAR_... environment variables to provide certain "global" settings to all configurations in your organization. To reduce the verbosity of these warnings, use the
│ -compact-warnings option.
╵
╷
│ Warning: Value for undeclared variable
│ 
│ The root module does not declare a variable named "project" but a value was found in file "terraform.tfvars". If you meant to use this value, add a "variable" block to the configuration.
│ 
│ To silence these warnings, use TF_VAR_... environment variables to provide certain "global" settings to all configurations in your organization. To reduce the verbosity of these warnings, use the
│ -compact-warnings option.
╵
╷
│ Error: Reference to undeclared input variable
│ 
│   on network.tf line 12, in resource "aws_vpc" "vpc":
│   12:     Name    = "${var.project}-${var.environment}-vpc"
│ 
│ An input variable with the name "project" has not been declared. This variable can be declared with a variable "project" {} block.
╵
╷
│ Error: Reference to undeclared input variable
│ 
│   on network.tf line 12, in resource "aws_vpc" "vpc":
│   12:     Name    = "${var.project}-${var.environment}-vpc"
│ 
│ An input variable with the name "environment" has not been declared. This variable can be declared with a variable "environment" {} block.
╵
╷
│ Error: Reference to undeclared input variable
│ 
│   on network.tf line 13, in resource "aws_vpc" "vpc":
│   13:     Project = var.project
│ 
│ An input variable with the name "project" has not been declared. This variable can be declared with a variable "project" {} block.
╵
╷
│ Error: Reference to undeclared input variable
│ 
│   on network.tf line 14, in resource "aws_vpc" "vpc":
│   14:     Env     = var.environment
│ 
│ An input variable with the name "environment" has not been declared. This variable can be declared with a variable "environment" {} block.
```

# network.tfに variable を追記した。
terraform.tfvars に書いてるやん！って思ったけど、こうやって呼び出して上げるお作法がないと動かないみたい。
```terraform
# ----------------------------------
# Create a VPC
# ----------------------------------
resource "aws_vpc" "vpc" {
  cidr_block                       = "192.168.0.0/20"
  instance_tenancy                 = "default"
  enable_dns_support               = true
  enable_dns_hostnames             = true
  assign_generated_ipv6_cidr_block = false

  tags = {
    Name    = "${var.project}-${var.environment}-vpc"
    Project = var.project
    Env     = var.environment
  }
}

variable "project" {
  type = string
}

variable "environment" {
  type = string
}
```