fix(nav): getLength is part of the public API

[liamdebeasi]

Issue number: resolves #28826

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What is the current behavior?

getLength used to be a public method on ion-nav. However, the @Method decorator was removed in 1d46973#diff-4b2ac275268173207c99a590b0ced8b67dc3d697815eb51a3c0546ee7cb0c0aeR429-L445.

While the removal of @Method from isAnimating was likely intentional, the team thinks that removing it from getLength was an accident. Otherwise, it's challenging to call methods like removeIndex if you don't know how many items are in the stack. For example, if a developer wants to remove the second to last item, they need to know how many items are in the stack so they don't give an index that is out of bounds.

What is the new behavior?

• getLength is now a public method

Does this introduce a breaking change?

• Yes
• No

The team is considering this to be a breaking change. As part of this, getLength must return Promise<number> instead of number as all public methods must return promises. While this API wasn't documented by mistake, it's technically always been a public API. As a result, JS-only developers can access getLength synchronously and TypeScript developers can do the same if they typecast Nav as any. Given that the return signature is different, we are going to ship this in a major release.

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