fix: Improve error messages thrown by iOS runtime

[mbektchiev]

• Use defaultSourceAppender for more detailed failed
  expression identification in thrown errors

• Change Function required error to match the actual validation
  check: Exactly 1 argument which must be a function required

• fix(interop): Reset pointer after free

  • Reassign deleted pointers to nullptr
  • Don't reuse cached pointer instances which have been freed

• refact: Don't throw in isValidType
  Make it a simple check whether its a valid type encoding or not
  and leave exception throwing to caller. It can construct a
  more meaningful error message because it has more context.

• fix(runtime): Improve error handling in overriding properties and methods

  • Add deepInstanceProperty and deepInstanceMethod getters in
    BaseClassMeta
  • Refactor _protocols field to use ProtocolMetas
  • Check and throw errors when overriding a native method with
    a JS property or a native property with a JS function

• chore: Add tests for error messages

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/ios-runtime/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.