## Description

This cleans up a lot of issues with the API doc generation.

Here are the main changes:
 - Rename `dartdoc.dart` to `create_api_docs.dart`
 - Move the bulk of the operations out of `dev/bots/docs.sh` into `create_api_docs.dart`.
 - Delete `dashing_postprocess.dart` and `java_and_objc.dart` and incorporate those operations into `create_api_docs.dart`.
 - Refactor the doc generation into more understandable classes
 - Bump the snippets tool version to 0.4.0 (the latest one)
 - Centralize the information gathering about the Flutter repo into the new `FlutterInformation` class.
 - Clean up the directory handling, and convert to using the `file` package for all file and directory paths.
 - Add an `--output` option to docs.sh that specifies the location of the output ZIP file containing the docs.
   - Defaults to placing the output in `dev/docs/api_docs.zip` (i.e. where the previous code generates the file).
 - Moved all document generation into a temporary folder that is removed once the documents are generated, to avoid VSCode and other IDEs trying to index the thousands of HTML and JS files in the docs output.
 - Updated pubspec dependencies.

## Tests
 - Added tests for doc generation.