* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

No code changes, just comments: I spell-checked all the comments in the repo.

Moves the logic for finding vcvars64.bat to a new VisualStudio class
that encapsulates finding, and providing information about, VisualStudio
installations. Adds a validator for it, and runs it for Windows
workflows in doctor.

Rather than hard-coding a set of locations to check, use vswhere (which
is installed by VS 2017 and later), and construct the vcvars64.bat path
relative to that. This will allow Windows builds to work without special
configuration for people who have VS installed at a custom path.

Also adds error logging with different messages for each failure point,
so that rather than the not-very-informative 'failed to find
vcvars64.bat' message, the failure will provide feedback about what to
do.

This is an interim solution; later this will be replaced by a
VisualStudio class with associated validator to match the structure of
the other toolchains.

Fixes #33249

Eliminates the need for a build.bat in the Windows build workflow, adding
preliminary support for building using msbuild. The handling of
vcvars64.bat may be refined in the future, but this serves as a starting point.