ref(core): Improved converter and function call parsing mechanism for XML expressions

[CatchABus]

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

What is the new behavior?

• Refactoring converter and function calls parsing mechanism
• Added missing exponentiation operator
• Fixed content inside template literals returning malformed value
• Support for using converters without using() just like in old NS
• Null-safety for parent properties whose value is null or undefined (similar to the Polymer way)