[roll] Roll fuchsia [fidlc] Implement modifier change syntax

This implements the modifier change syntax from "Amendments to RFC-0232:
FIDL bindings for multiple API levels". So instead of this:

    @available(replaced=5)
    type Foo = strict enum {
        // 100 lines ...
    };

    @available(added=5)
    type Foo = flexible enum {
        // copy & paste the 100 lines ...
    };

You can now write this:

    type Foo = strict(removed=5) flexible(added=5) enum {
        // 100 lines ...
    };

The same syntax works with `resource`, `closed`, `ajar`, and `open`.

The implementation works by treating modifiers as elements, and reusing
all the existing code for @available. I had to refactor some things
because modifier values are now not known until after AST decomposition.
This is particularly annoying for method result unions, whose existence
depends on method strictness. I decided to always create them, and
remove the framework error later on if the method is strict. Removing
the entire union proved too difficult, so I worked around that by
disallowing changing strictness on two way methods without error syntax,
which is not useful anyway because it's ABI breaking.

This change regresses the ability to define a method named "strict" or
"flexible", since supporting that with this new syntax would require
more lookahead in the parser. Once we ratify and implement explicit
method kinds (https://fxbug.dev/348403275), there will be no ambiguity
and this will be fixed.

We might want to extend the fidl-lint rule for explicit modifiers to
ensure there are no gaps when using this syntax. In the example code
above, you can omit `flexible(added=5)` since `flexible` is the default.
Currently, the linter would not complain about that because it only
operates on the raw AST, making it hard to write that kind of check.

Test: fx test fidlc-test
Test: fx check-goldens fidl
Test: fx format-code --files=(git ls-files '*.fidl' | tr '\n' ,)
Original-Fixed: 348405702 (modifier change syntax)
Original-Fixed: 42063565 (incorrect error for unnamed attribute arg)
Original-Bug: 348403275 (explicit method kinds)
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1068250
Original-Revision: 7a91bcda60ae659615dd4191c0243bcb66b0d37c
GitOrigin-RevId: e573ed76b4cbdd8ba30548d5c775906a197c10da
Change-Id: I3d0390d19b88540d94f18e3b1611474f67f1440e