<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
Every lambda capture in `Tab` and `TerminalPage` has been changed from capturing raw `this` to `std::weak_ptr<Tab>` or `winrt::weak_ref<TerminalPage>`. Lambda bodies have been changed to check the weak reference before use.

Capturing raw `this` in `Tab`'s [title change event handler](https://github.com/microsoft/terminal/blob/master/src/cascadia/TerminalApp/Tab.cpp#L299) was the root cause of #3776, and is fixed in this PR among other instance of raw `this` capture.

The lambda fixes to `TerminalPage` are unrelated to the core issue addressed in the PR checklist. Because I was already editing `TerminalPage`, figured I'd do a [weak_ref pass](#3776 (comment)).

<!-- Other than the issue solved, is this relevant to any other issues/existing PRs? -->

<!-- Please review the items on the PR checklist before submitting-->
* [x] Closes #3776, potentially #2248, likely closes others
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Requires documentation to be updated
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #3776

<!-- Provide a more detailed description of the PR, other things fixed or any additional comments/features here -->
`Tab` now inherits from `enable_shared_from_this`, which enable accessing `Tab` objects as `std::weak_ptr<Tab>` objects. All instances of lambdas capturing `this` now capture `std::weak_ptr<Tab>` instead. `TerminalPage` is a WinRT type which supports `winrt::weak_ref<TerminalPage>`. All previous instance of `TerminalPage` lambdas capturing `this` has been replaced to capture `winrt::weak_ref<TerminalPage>`. These weak pointers/references can only be created after object construction necessitating for `Tab` a new function called after construction to bind lambdas.

Any anomalous crash related to the following functionality during closing a tab or WT may be fixed by this PR:
- Tab icon updating
- Tab text updating
- Tab dragging
- Clicking new tab button
- Changing active pane
- Closing an active tab
- Clicking on a tab
- Creating the new tab flyout menu

Sorry about all the commits. Will fix my fork after this PR! 😅

<!-- Describe how you validated the behavior. Add automated tests wherever possible, but list manual validation steps taken as well -->
Attempted to repro the steps indicated in issue #3776 with the new changes and failed. When before the changes, the issue could consistently be reproed.

(cherry picked from commit 7b9728b)