Add support for Cloudflare's per-record CNAME flattening feature using
CF_CNAME_FLATTEN_ON and CF_CNAME_FLATTEN_OFF modifiers, similar to the
existing CF_PROXY_ON/OFF pattern.

CNAME flattening resolves CNAME targets to their IP addresses at
Cloudflare's edge, which can improve performance and compatibility.
This feature requires a paid Cloudflare plan (Pro, Business, or
Enterprise).

New record modifiers for CNAME records:
- CF_CNAME_FLATTEN_ON: Enable CNAME flattening for the record
- CF_CNAME_FLATTEN_OFF: Disable CNAME flattening (default)

Example usage:
```javascript
D("example.com", REG, DSP,
    CNAME("cdn", "cdn.provider.com.", CF_CNAME_FLATTEN_ON),
    CNAME("www", "www.example.com.", CF_CNAME_FLATTEN_OFF),
);
```

- Uses Cloudflare API's `settings.flatten_cname` field (not tags, as
  the `cf-flatten-cname` tag is reserved by Cloudflare)
- Stores state in record metadata as `cloudflare_cname_flatten`
- Only applies to CNAME record types
- Integrated with diff detection via genComparable()

- pkg/js/helpers.js: Add CF_CNAME_FLATTEN_ON/OFF constants
- commands/types/others.d.ts: Add TypeScript declarations for IDE support
- providers/cloudflare/cloudflareProvider.go:
  - Add metaCNAMEFlatten constant
  - Add checkCNAMEFlattenVal() validation function
  - Update preprocessConfig() to validate CNAME flatten setting
  - Update nativeToRecord() to read settings.flatten_cname from API
  - Update genComparable() to include flatten state in diff comparison
- providers/cloudflare/rest.go:
  - Update createRecDiff2() to set Settings.FlattenCNAME
  - Update modifyRecord() to set Settings.FlattenCNAME
- commands/getZones.go:
  - Update TSV export to include cloudflare_cname_flatten metadata
  - Update JS/DJS export to include CF_CNAME_FLATTEN_ON modifier
- pkg/prettyzone/prettyzone.go:
  - Update zone file export to include CF_CNAME_FLATTEN_ON comment

Added gated integration tests that are disabled by default since CNAME
flattening requires a paid plan:
- integrationTest/helpers_test.go: Add -cfflatten flag (default false)
- integrationTest/helpers_integration_test.go: Add test helper functions
- integrationTest/integration_test.go: Add CF_CNAME_FLATTEN test groups

Run tests on paid zones with: go test -v -provider=CLOUDFLAREAPI -cfflatten=true

Updated documentation/provider/cloudflareapi.md with:
- New CNAME flattening section with usage examples
- Warning about paid plan requirement
- Integration test flag documentation