[cfe,analyzer] Share experimental flags for the parser

This adds a new generated file into _fe_analyzer_shared for the
experimental flags. This is now used by the parser when generating
messages about features not being available.

The default implementation uses whether the feature is enabled by default
or not, to emit different messages. This change will therefore ensure
that when a missing feature changes from 'experimental' to
'enabled by default', the  message reported by the parser will be
updated accordingly. Currently errors are only reported from the parser
on features that are enabled by default, so the message for experimental
features is not currently used.

The reporting is performed through the Listener, such that
implementations can override the reporting to improve the messaging.
This is done in the CFE where the message is improved to take into
account whether the language version is explicit in the parsed library.

In response to https://github.com/dart-lang/sdk/issues/46329

Change-Id: Ief812817c7eb4b1e433389f6f49d6a1f77604fa7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/269860
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>