append collection function

### Current Terraform Version

```
v0.12.24
```

### Use-cases

It would be nice to be able to append items to a list, instead of creating an arbitrary list simply to use the `concat()` function

### Attempted Solutions

To add items to a list, you must create a new list and use the `concat()` function.

```hcl
variable "input_list" {
type = list(string)
default = ["one", "two", "three"]
}

locals {
new_value = "four"
merged_list = concat(var.input_list, [local.new_value])
}
```

### Proposal

A new `apppend()` function that takes a list and a value, and adds the value to the list.
```hcl
variable "input_list" {
type = list(string)
default = ["one", "two", "three"]
}

locals {
new_value = "four"
merged_list = append(var.input_list, local.new_value)
}
```

### References