Improve non-top-level import/export error, especially for JS

[sandersn]

ES imports and exports can only be used at the top level of a module. This is illegal:

function container() {
  import 'fs'
  export { container }
  namespace N { }
}

The current errors for these three statements are vague and, for JS, contain irrelevant terms:

Actual:

(1) "An import declaration can only be used in a namespace or module."
(2) "An export declaration can only be used in a module."
(3) "A namespace declaration is only allowed in a namespace or module."

Expected:

(1) When the node is in a JS file, "An import declaration can only be used at the top level of a module."
Otherwise, "An import declaration can only be used at the top level of a namespace or module."
(2) When the node is in a JS file, "An export declaration can only be used at the top level of a module."
Otherwise, "An export declaration can only be used at the top level of a namespace or module."
(3) "A namespace declaration is only allowed at the top level of a module."

Implementation:
checkGrammarModuleElementContext issues these errors. I don't know whether it's better to make it smarter or just avoid calling it for case (1).

[fatcerberus]

Isn’t export used inside namespaces too? So (2) should probably be “An export declaration can only be used at the top level of module or namespace.”

[vicente-s]

Hey everyone, I'd like to take on this issue.

[lokicodedaily]

can I work on this issue?

[sandersn]

@lokicodedaily sure, although you might want to co-ordinate with @vicente-s.

[sandersn]

@fatcerberus Good point. I updated the Expected section.

[islandryu]

@sandersn
I tried to fix it in #47087, but did I do something wrong in the PR procedure?
I would be grateful for any feedback.

[sandersn]

@islandryu I'm backlogged on community PRs. I'll review it when I can.

[DanielRosenwasser]

Thank you @islandryu!