Remove dependency on third party package:charcode.

The charcode package used to be first party but has become a third party
package. This change inlines the needed constants from package:charcode
in the style done by other packages that used to depend on it.

This change is being done to simplify the security review of upgrading
the Dart SDK DEPS as package:charcode is the only third party package in
there and it's barely used in the SDK. package:markdown is the last
package in the SDK DEPS to depend on package:charcode.

Bug: b/230318198