Fix extension validation for proto2 required fields

[doriable]

This PR adds validation for non-editions features required
fields on extensions.

[emcfarlane]

label the right term here, what about a test for LEGACY_REQUIRED? extensions can't be required. would maybe fit both patterns better.

[doriable]

The LEGACY_REQUIRED goes through the features, so the output you get is:

expected singular field, found extension

Which is basically the unexpected error... I had considered making them as similar as possible, in terms of the diagnostics, but I wasn't sure if that made sense in this case? So I just kind of kept them as separate diagnostics...

[emcfarlane]

Would be nicer to make it explicit.

[doriable]

And as for the test case with LEGACY_REQUIRED -- I can add a test case, but proto2 files cannot set features, so it's not really a problem right now:

            error: `features` cannot be set in "proto2"
              --> testdata/extend/required.proto:16:33
               |
             1 | syntax = "proto2";
               |          -------- syntax specified here
            ...
            16 |     required string meow = 103 [features.field_presence = LEGACY_REQUIRED];
               |                                 ^^^^^^^^

            error: `field_presence` is not supported in "proto2"
              --> testdata/extend/required.proto:16:33
               |
             1 | syntax = "proto2";
               |          -------- syntax specified here
            ...
            16 |     required string meow = 103 [features.field_presence = LEGACY_REQUIRED];
               |                                 ^^^^^^^^^^^^^^^^^^^^^^^
               |
               = help: `field_presence` requires at least Edition 2023

[doriable]

Would be nicer to make it explicit.

So the full diagnostic is explicit, it has help statement that says field_present cannot be set on extensions:

protocompile/experimental/ir/testdata/options/validate/presence.proto.stderr.txt

Lines 90 to 99 in bb5762d

	error: expected singular field, found extension
	--> testdata/options/validate/presence.proto:26:5
	|
	26 | int32 e = 4 [features.field_presence = EXPLICIT];
	| ^^^^^^^^^^^^^-----------------------^^^^^^^^^^^^^
	| |
	| `field_presence` set here
	|
	= help: `field_presence` cannot be set on extensions
	= help: all singular extensions have explicit presence

The output I provided is from buf, which currently doesn't use the full diagnostic report, so it doesn't appear to be explicit... but yeah...

[emcfarlane]

Suggested change

	d.Apply(report.Snippet(required.PrefixToken()))
	} else {
	d.Apply(report.Snippet(m.AST()))
	}); !required.IsZero() {
	d.Apply(report.Snippet(required.PrefixToken()))
	d.Apply(report.Helpf("extensions cannot use the `required` label"))
	} else {
	d.Apply(report.Snippet(m.AST()))
	d.Apply(report.Helpf("extensions cannot set `field_presence` to `LEGACY_REQUIRED`"))
	}

[doriable]

^ re: conversation in comment above, this is only checking for required field, and the features.field_presence is checked below with different diagnostics, so we shouldn't mix the diagnostics here.