[new syntax] Support `function` `ref` Type Parameters for function pointer types

[ntrel]

Fixes Issue 2753 - Cannot declare pointer to function returning ref.

This token ordering is consistent with function literals:

function RefOrAutoRefopt Typeopt ParameterWithAttributesopt FunctionLiteralBody2

https://dlang.org/spec/expression.html#function_literals

See https://forum.dlang.org/post/[email protected] for more on the grammar.

Alternatively, allowing Type function Parameters ref would also fix the issue, though that is less easy to see that the return value is actually a reference when looking at the return type. Also there is no precedent for that AIUI in the grammar, though the compiler does use this ordering in error messages despite it being invalid syntax.

Another option would be Type ref function Parameters, but again, no precedent for that.

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This pull implements support for 'leading function' pointer types in most places, ~~but not for a declaration at module scope yet~~.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	2753	enhancement	Cannot declare pointer to function returning ref

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#15243"

[ntrel]

Although this solves the ref problem, there is another syntax ambiguity with scope if we want to extend dip1000. See: https://forum.dlang.org/post/[email protected]

That problem is not solved by this pull alone.

Edit: Turns out we don't need scope there.