Report & generate missing type constraint in `variable` block

[radeksimko]

Context

Users may - and anecdotally often do - write configuration without considering the type of variables, for example:

variable "foo" {
}

Sometimes they go one step further but still not very far by defining the type as any:

variable "foo" {
  type = any
}

or they may let the type to be inferred from the default value, e.g.

variable "foo" {
  default = []
}

In all these cases, they can introduce bugs due to type mismatches/conversions or at least make it much more difficult for collaborators and consumers of the module to understand the module as a whole.

Proposal

• Raise missing type constraint as a warning diagnostic, suggesting that types are better declared explicitly to communicate intentions
• Raise type = any as informal diagnostic, suggesting more concrete type, to communicate intentions
• Provide quickfix code action to generate type based on default value