Only sync Glean.js

[rosahbruno]

This PR updates Glean.js to be completely synchronous with only a web implementation.

The 3 other platforms are going to undergo changes

• QT was deprecated in Remove QT #1810
• Node is being temporarily removed in Remove current node implementation #1813
• Web Extension is being temporarily removed in Remove current webext implementation #1814

Both the node and web extension platforms are going to be added back in later. They are being removed now so we can focus on web and build a new architecture that allows for platform OR framework specific implementations with their own auto-instrumentation.

Important updates included in this PR

• Removal of async/sync file structure
• Removal of the dispatcher since it is not needed in the sync implementation
• Removal of plugins as they are asynchronous and not used in any active projects
• Removal of tests that are async specific and no longer fit in Glean.js

[Dexterp37]

@rosahbruno a few observations on this:

• Please remove the "builtin metrics" from this PR. We should tackle this as a follow-up after this refactor.
• Please break out the QT code removal out of this PR, to simplify reviewing (instead of removing the docs from the Glean Book, we should mark that as deprecated, mention the last supported version and file a bug for moving these docs to an "Obsolete" part of the book) - Update: Took a stab at this in Remove QT #1810

Note that I see 13 unit tests failing locally, not 2:

• Some of them are failing because of isWindowObjectUnavailable in internal_metrics.ts/initialize
• Some of them are failing because of the newly automatically instrumented page load events (e.g. "send the 'events' ping when max capacity is hit")

By splitting the PR as advised above we should be more confident in the changes we make.

[rosahbruno]

@rosahbruno a few observations on this:

* Please remove the "builtin metrics" from this PR. We should tackle this as a follow-up after this refactor.

* Please break out the QT code removal out of this PR, to simplify reviewing (instead of removing the docs from the Glean Book, we should mark that as deprecated, mention the last supported version and file a bug for moving these docs to an "Obsolete" part of the book)

Yep, this makes sense. I will break these into separate PRs.

Note that I see 13 unit tests failing locally, not 2:

* Some of them are failing because of `isWindowObjectUnavailable` in internal_metrics.ts/initialize

* Some of them are failing because of the newly automatically instrumented page load events (e.g. `"send the 'events' ping when max capacity is hit"`)

By splitting the PR as advised above we should be more confident in the changes we make.

I will take a look

[rosahbruno]

This PR is now rebased and includes the changes from #1810. It also includes and builds off the changes made in #1813 and #1814.

I am going to clean up these commits so they are more readable and easier to review.

[travis79]

No blocking comments or arguments from me. That was a lot of await and async to remove!