feat: Refactor and update CLI package

[inferrinizzard]

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[schani]

Thanks for this — splitting src/index.ts into focused modules (cli.options.ts, quicktype.options.ts, sources.ts, etc.) is a welcome direction. A few things need to be resolved before this can merge:

Hard requirements

• src/utils.ts's negatedInferenceFlagName now uses lodash's _.capitalize, which (unlike quicktype-core's capitalize in packages/quicktype-core/src/support/Strings.ts) lowercases the rest of the string, not just the first character. Since the negated flag name feeds dashedFromCamelCase, this silently renames CLI flags: --no-date-times → --no-datetimes, --no-integer-strings → --no-integerstrings, --no-boolean-strings → --no-booleanstrings (both in parsing and in --help output). No fixture/unit test covers these flags, so CI stayed green. Please keep quicktype-core's capitalize.
• The new src/index.ts only exports writeOutput, main, and the CLIOptions type. parseCLIOptions (now in cli.options.ts), makeQuicktypeOptions (quicktype.options.ts), and jsonInputForTargetLanguage (input.ts) are no longer re-exported from the package entry point (package.json's main/types still point at dist/index.js/dist/index.d.ts). That's a silent removal of public API for programmatic consumers of the quicktype npm package. Since we're planning a major version, dropping these deliberately is negotiable — dropping them silently isn't. Please either re-export them or call out the removal explicitly.

Please also address

• The branch has real conflicts with current master in src/GraphQLIntrospection.ts, src/index.ts, test/languages.ts, test/lib/deepEquals.ts, test/lib/multicore.ts, and test/test.ts — master has since moved on (e.g. the boolean --help fix, integer-range inference, the Biome formatting pass, and test/lib/multicore.ts now using process.exit instead of the removed exit package). Because this PR relocates index.ts wholesale, resolving these amounts to redoing the split against current master.
• New file names (cli.options.ts, quicktype.options.ts, CLIOptions.types.ts, and friends) don't match the repo's existing module convention (TypeSource.ts, URLGrammar.ts, GraphQLIntrospection.ts) — please align on PascalCase.
• This PR's file set is a subset of your own chore: Add unit tests for options functions and cli #2792, which touches the same files plus packages/quicktype-core and adds unit tests. We won't merge both — let's consolidate into a single PR, redone on current master, with the two hard requirements above addressed.