refactor(rome_diagnostics): remove the legacy Diagnostics API

[leops]

Summary

The old rome_diagnostics API isn't being used by anything anymore, so this PR removes it. Most of the changes come from moving the previous v2 namespace to the crate root, which implies rewriting pretty much every import of rome_diagnostics.
The only 2 parts of the previous API still in use are the CodeSuggestion struct (this remains in the suggestion module of rome_diagnostics) and the Span trait (this was moved to the span module of rome_js_parser since it's only used there and could be confused with the AsSpan trait exposed by the new diagnostics API).

Test Plan

This is purely a type-level change with no modification to the logic of the code, so all of it should be checked by rustc at compile time.

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Parser conformance results on ubuntu-latest

js/262

Test result	main count	This PR count	Difference
Total	45879	45879	0
Passed	44936	44936	0
Failed	943	943	0
Panics	0	0	0
Coverage	97.94%	97.94%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	39	39	0
Passed	36	36	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.31%	92.31%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	5946	5946	0
Passed	1621	1621	0
Failed	4325	4325	0
Panics	0	0	0
Coverage	27.26%	27.26%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	588	588	0
Passed	519	519	0
Failed	69	69	0
Panics	0	0	0
Coverage	88.27%	88.27%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	16257	16257	0
Passed	12397	12397	0
Failed	3860	3860	0
Panics	0	0	0
Coverage	76.26%	76.26%	0.00%