Extract Terraform extension (#10479)

This PR extracts Terraform support into an extension and removes the
built-in Terraform support from Zed.

Release Notes:

- Removed built-in support for Terraform, in favor of making it
available as
an extension. The Terraform extension will be suggested for download
when you
open a `.tf`, `.tfvars`, or `.hcl` file.

extensions/terraform/languages/terraform-vars/injections.scm

@@ -0,0 +1,9 @@
+; https://github.com/nvim-treesitter/nvim-treesitter/blob/ce4adf11cfe36fc5b0e5bcdce0c7c6e8fbc9798a/queries/hcl/injections.scm
+
+(heredoc_template
+  (template_literal) @content
+  (heredoc_identifier) @language
+  (#downcase! @language))
+
+; https://github.com/nvim-treesitter/nvim-treesitter/blob/ce4adf11cfe36fc5b0e5bcdce0c7c6e8fbc9798a/queries/terraform/injections.scm
+; inherits: hcl