Feat: add terraform template filetype

[ProbstDJakob]

Is your feature request related to a problem? Please describe.
When writing terraform files, there is sometimes the need to write large template strings like config files and those are best split out into their own files. Terraform does support this by the templatefile function which recommends using .tftpl as suffix. Currently there is no way to parse such files and therefore neovim does not highlight those files.

Describe the solution you'd like
It would be nice to have a terraform-template or hcl-template dialect with the possibility to inject a language like with the go template tree-sitter plugin.

Describe alternatives you've considered
No highlighting/syntax tree.