refactor(manager/gradle): group parser directives in separate files

[Churro]

Changes

• parser.ts: split the functionality into dedicated files (pure copy-paste without functional changes) for better maintainability. Where meaningful, directives were exported, summarized as qDependencies or qAssignments to abstract away individual Groovy and Kotlin constructs from parser.ts.
• handler.ts: Apply a previous refactoring suggestion regarding ctx.globalVars to all occurrences

Context

• feat(manager/gradle): reimplement parser using tree-based approach #18663 (comment)
• feat(manager/gradle): add support for Groovy and Kotlin maps #19127 (comment)

Documentation (please check one with an [x])

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• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This PR is included in version 34.63.2 🎉

The release is available on:

• GitHub release
• 34.63.2

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