fix: correct decorator traversal for AssignmentPattern

[bradzacher]

Fixes #2369

BREAKING CHANGE:

• Removed decorators property from several Nodes that could never semantically have them (FunctionDeclaration, TSEnumDeclaration, and TSInterfaceDeclaration)
• Removed AST_NODE_TYPES.Import. This is a minor breaking change as the node type that used this was removed ages ago.

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This was an interesting bug to fix!
We had a bug in our visitor keys here - they didn't declare the decorators key for AssignmentPattern, this meant that when ESLint traversed the tree, it wouldn't traverse assignment pattern's decorators, so it wouldn't add the parent property.

So when no-unused-vars attempted to get the parent of the Optional Identifier for the decorator, it wouldn't find one, and hence it would crash.

This PR:

• adds the missing visitor keys for AssignmentPattern
• removes decorator keys for nodes that aren't semantically allowed to have them.
  • I'm not sure why we ever bothered emitting invalid code here because:
    • TS will parse-time allow it, but will semantic error. And then it scrubs the decorator from compiled code, so the decorator is 100% invalid in all these cases.
    • Babel will parse-time error.
• removes AST_NODE_TYPES.Import
• adds a funky typecheck-time test to ensure all AST_NODE_TYPES members have an associated TSESTree.Node type (so we don't forget to delete them ever again)
• adds strict types to vistor-keys so that IDEs give autocomplete, and we ensure that both the array of keys is valid, and that the node types all exist in AST_NODE_TYPES.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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[codecov]

Codecov Report

Merging #2375 into v4 will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##               v4    #2375      +/-   ##
==========================================
- Coverage   93.00%   92.96%   -0.04%     
==========================================
  Files         286      286              
  Lines        9060     9038      -22     
  Branches     2523     2513      -10     
==========================================
- Hits         8426     8402      -24     
- Misses        304      305       +1     
- Partials      330      331       +1     

Flag	Coverage Δ
#unittest	92.96% <66.66%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...kages/eslint-plugin/src/rules/no-empty-function.ts	77.14% <0.00%> (-6.65%)	⬇️
...nt-plugin/src/rules/no-unused-vars-experimental.ts	91.48% <ø> (ø)
packages/typescript-estree/src/convert.ts	98.34% <ø> (-0.02%)	⬇️
...pt-estree/src/ts-estree/estree-to-ts-node-types.ts	100.00% <ø> (ø)
packages/visitor-keys/src/visitor-keys.ts	100.00% <100.00%> (ø)
...ckages/eslint-plugin/src/rules/no-throw-literal.ts	96.07% <0.00%> (-0.85%)	⬇️