Support Terraform Variable Definitions as separate language (#7524)

With https://github.com/zed-industries/zed/pull/6882 basic syntax
highlighting support for Terraform has arrived in Zed. To fully support
all features of the language server (when it lands), it's necessary to
handle `*.tfvars` slightly differently.

TL;DR: [terraform-ls](https://github.com/hashicorp/terraform-ls) expects
`terraform` as language id for `*.tf` files and `terraform-vars` as
language id for `*.tfvars` files because the allowed configuration
inside the files is different. Duplicating the Terraform language
configuration was the only way I could see to achieve this.

---

In the
[LSP](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocumentItem),
text documents have a language identifier to identify a document on the
server side to avoid reinterpreting the file extension.

The Terraform language server currently uses two different language
identifiers:
* `terraform` - for `*.tf` files
* `terraform-vars` - for `*.tfvars` files

Both file types contain HCL and can be highlighted using the same
grammar and tree-sitter configuration files. The difference in the file
content is that `*.tfvars` files only allow top-level attributes and no
blocks. [_So you could argue that `*.tfvars` can use a stripped down
version of the grammar_]. To set the right context (which affects
completion, hover, validation...) for each file, we need to send a
different language id.

The only way I could see to achieve this with the current architecture
was to copy the Terraform language configuration with a different `name`
and different `path_suffixes`. Everything else is the same.

A Terraform LSP adapter implementation would then map the language
configurations to their specific language ids:

```rust
fn language_ids(&self) -> HashMap<String, String> {
    HashMap::from_iter([
        ("Terraform".into(), "terraform".into()),
        ("Terraform Vars".into(), "terraform-vars".into()),
    ])
}
```

I think it might be helpful in the future to have another way to map
file extensions to specific language ids without having to create a new
language configuration.

### UX Before

![CleanShot 2024-02-07 at 23 00
56@2x](https://github.com/zed-industries/zed/assets/45985/2c40f477-99a2-4dc1-86de-221acccfcedb)

### UX After

![CleanShot 2024-02-07 at 22 58
40@2x](https://github.com/zed-industries/zed/assets/45985/704c9cca-ae14-413a-be1f-d2439ae1ae22)

Release Notes:

- N/A

---

* Part of https://github.com/zed-industries/zed/issues/5098

crates/zed/src/languages.rs

@@ -287,6 +287,7 @@ pub fn init(
     language("uiua", vec![Arc::new(uiua::UiuaLanguageServer {})]);
     language("proto", vec![]);
     language("terraform", vec![]);
+    language("terraform-vars", vec![]);
     language("hcl", vec![]);
 }