Finalize the definition link api

- Gives fields more explicit names (target and origin)
- Moves api to vscode.d.ts
- Makes other definition providers (such as type definition provider and implementation provider) also return definition links

Fixes #54101

Xamarin Workbooks are interactive coding documents that are saved as
straight-forward markdown files with a YAML front matter header block.

Here is a sample: https://github.com/xamarin/Workbooks/blob/master/csharp/csharp6/csharp6.workbook

Github has been treating them as markdown files for over a year now
(https://github.com/github/linguist/pull/3500).

Fixes https://github.com/Microsoft/vscode-pull-request-github/issues/58
Fixes https://github.com/Microsoft/vscode-pull-request-github/issues/59

Block quote in comments styling, fixes https://github.com/Microsoft/vscode-pull-request-github/issues/60

Fixes #53423

- Returned undefined instead of empty array
- Only execute server call in try catch

This API has been replaced with navbar. The code related to navbar is not being tested and a very small number of users are using < 2.1 in their workspaces

We want to be alerted if an exception is thrown outside of execute

* Open release notes in product from changelog in built in extensions

* Allow only the release notes command from webview

* Localized text may invalidate markdown, skip it

Fix handling of links in comment panel, fixes https://github.com/Microsoft/vscode-pull-request-github/issues/92

fixes #54486

Part of #54510

This reverts commit e587012b.

fixes #54486