fix: add meta for TextBase and FormattedString

[edusperoni]

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.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

We currently fallback to _addChildFromBuilder for spans

What is the new behavior?

We now implement a meta that correctly implements add/remove child spans

Fixes #65

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