feat(framework): add .currentTarget to the type of event handler in TSX and UI5CustomEvent

[pskelin]

The generated event handlers for events described in eventDetails now have the correct type for the e.currentTarget parameter

<Input onClick={(e => e.currentTarget)} />
                     // ^^^^^^^^^^^^^
                     // `ui5-input` element now, was `EventTarget` before

Additionally, the UI5CustomEvent helper also adds the correct type to the event parameter e.currentTarget

An event handler no longer has to use type assertions

Before:

onInput(e: CustomEvent) {
  const value: string = (e.target as Input).value;

This works, but the same event handler is also attached to a ui5-slider and the e.target.value is a number in that case which goes unnoticed because of the as Input type assertion.

After

onInput(e: UI5CustomEvent<Input, "input">) {
  const value: string = e.currentTarget.value;
//                        ^^^^^^^^^^^^^^^^^^^
//                        this is checked now, adding the same handler to a slider will result in an error

Implementation note

Event handlers are contravariant (an event handler that takes a subclass as a parameter cannot be assigned to a property in the base class that takes the base class as a parameter).

To circumvent this, the generated event handlers are using the method notation so that typescript handles them as bivariant

More info in the strictFunctionTypes compiler option here:
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-6.html#strict-function-types

[ui5-webcomponents-bot]

🎉 This PR is included in version v2.8.0 🎉

The release is available on v2.8.0

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