…al_link file

This was preventing us from making private `JsonEnum` classes
as the generated `EnumMap` does not follow the rule of
being a lowerCamelCase identifier.

For example, `_Foo` generates `_$_FooEnumMap` and was
triggering an analyzer complaint.

The core process on parsing internal links (here in
`lib/model/internal_link.dart`) relied heavily on the
existing code in the Zulip mobile app - from
`src/utils/internalLinks.js`. In fact the `_parseStreamOperand`
function here is a line for line port in order to capture the
same semantics when processing streams.

Where the implementation differs is this new process is less
restrictive on the order of operator/operand pairs:
supporting `#narrow/topic/_/stream_` where mobile only
accepted `#narrow/stream/_/topic/_`.

Also, the mobile implementation accepted as valid narrows
DM operators with an email address as the operand
(`#narrow/dm/a.40b.2Ecom.2Ec.2Ed.2Ecom`) but created an
invalid narrow object (with NaNs for targets) whereas this
implementation rejects them as invalid narrows.

Likewise the test cases are also taken from the mobile code
(`src/utils/__tests__/internalLinks-test.js`) and replicated
here, save for the special narrow types (`#narrow/is/starred`)
which are not yet implemented. Also, the "without realm info"
cases were removed as they were made moot with every test
case being passed through `tryResolveOnRealmUrl` (the mobile
cases were also passed through `new Url()` with a base).