[_fe_analyzer_shared] Code generate ScannerErrorCode class.

I suspect that the reason this wasn't previously code generated was
because even though it follows the conventions of the other analyzer
error code classes, and is used by the analyzer, it exists in
`pkg/_fe_analyzer_shared`, since it is used by the scanner.

In order for this to be code generated without creating a reverse
dependency from `_fe_analyzer_shared` to `analyzer`, I had to change
the code generator so that the generated files import
`package:_fe_analyzer_shared/src/base/errors.dart` rather than
`package:analyzer/error/error.dart`. This is a benign change so I went
ahead and let it apply to all code generated by the analyzer error
message code generator, even though it's only strictly necessary for
code generated into the `_fe_analyzer_shared` package.

Since code in `pkg/_fe_analyzer_shared` is required to use explicit
`new` or `const` when constructing class instances, and since this
coding convention is enforced by a custom CFE-based lints that doesn't
understand `ignore:` comments, I also had to modify the code generator
to insert the appropriate `const` keywords when generating the new
code. To reduce churn and confusion, I added the `const` keywords only
to error code classes inside `pkg/_fe_analyzer_shared`.

I also had to improve the code generator so that it properly escapes
`$` if it appears inside an error message. Previously, this was not a
problem, because there were no code generated error messages that
contained `$`.

In the process of making this change, I discovered that entries for
`ScannerErrorCode.UNEXPECTED_SEPARATOR_IN_NUMBER` were missing from
`error_fix_status.yaml` and from the `diagnosticCodeValues` list. I've
added the missing entries as part of the CL.

Change-Id: Ib6e734a47e77b406059df9cfdeff9e401f22bd88
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/439060
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>