refactor: Use DSL v2 types in place of ScannerDefinitionNode

[Xanewok]

Based on #1002
Part of #638

The DSL v2's model::Scanner is almost 1:1 usable with our current parser codegen, with the exception of versioned scanners. In short we:

• introduce a helper type VersionedScanner that is used in place of the old ScannerDefinitionNode::Versioned
• introduce a ScannerExt trait implemented for both VersionedScanner and the model::Scanner, which is responsible for generating the main scanning logic
• Repurposes ScannerDefinition slightly to surface more of the scanner-related logic for trivia/fragment/token
• implements this directly for model::{TriviaItem,FragmentItem,TokenItem} and stores it in the v1 Grammar struct
• similarly, uses model::KeywordItem directly in place of v1's KeywordScannerDefinition as they shared the same functionality

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[OmarTawfik]

Left one suggestion. But I think @AntonyBlakey would be a better reviewer for the main changes here.

[AntonyBlakey]

I'm not clear about the separation between Keyword and Scanner - one is a verb and one a noun, but I don't know if that is true about their roles? This split happened after I left the codebase, so it's my responsibility I know to work it out, but TBH the naming is becoming less self-describing and more an in-group code/shorthand.

[Xanewok]

@AntonyBlakey I've changed some names, let me know if that's okay now 🙏

[AntonyBlakey]

Not 100% happy with the naming in this section of our codebase, but this PR is not the place for that more general argument, so I am approving this now.