Add Syntax Highlighting for for_each, count, and dynamic

[hariscodes]

It would increase readability if we added highlighting for for_each, count, and dynamic keywords. The extention already does this for for, we can use the same highlighting for these other non-function keywords as well.

[kquinsland]

And nullable in the variable block: https://developer.hashicorp.com/terraform/language/values/variables#disallowing-null-input-values