Add combine_flatmap lint rule

[ldct]

Summary

• Adds a new combine_flatmap lint rule that warns against using Combine's flatMap operator
• Detects both Combine.flatMap { ... } call expressions and Combine.flatMap member access references
• Includes non-triggering examples for array flatMap and Combine.map to avoid false positives

Test plan

• Verify non-triggering examples pass (array flatMap, Combine.map)
• Verify triggering examples flag violations (Combine.flatMap call and member access)
• Run swift run swiftlint-dev rules validate to confirm rule registration

🤖 Generated with Claude Code

[SwiftLintBot]

	1 Warning
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

	19 Messages
📖	Building this branch resulted in a binary size of 27261.38 KiB vs 27252.48 KiB when built on main (0% larger).
📖	Linting Aerial with this PR took 0.81 s vs 0.8 s on main (1% slower).
📖	Linting Alamofire with this PR took 1.05 s vs 1.06 s on main (0% faster).
📖	Linting Brave with this PR took 7.2 s vs 7.23 s on main (0% faster).
📖	Linting DuckDuckGo with this PR took 25.09 s vs 25.16 s on main (0% faster).
📖	Linting Firefox with this PR took 11.64 s vs 11.64 s on main (0% slower).
📖	Linting Kickstarter with this PR took 7.99 s vs 8.02 s on main (0% faster).
📖	Linting Moya with this PR took 0.47 s vs 0.47 s on main (0% slower).
📖	Linting NetNewsWire with this PR took 2.48 s vs 2.47 s on main (0% slower).
📖	Linting Nimble with this PR took 0.71 s vs 0.7 s on main (1% slower).
📖	Linting PocketCasts with this PR took 7.55 s vs 7.67 s on main (1% faster).
📖	Linting Quick with this PR took 0.48 s vs 0.45 s on main (6% slower).
📖	Linting Realm with this PR took 3.25 s vs 3.33 s on main (2% faster).
📖	Linting Sourcery with this PR took 1.9 s vs 1.89 s on main (0% slower).
📖	Linting Swift with this PR took 4.43 s vs 4.42 s on main (0% slower).
📖	Linting SwiftLintPerformanceTests with this PR took 0.34 s vs 0.37 s on main (8% faster).
📖	Linting VLC with this PR took 1.18 s vs 1.15 s on main (2% slower).
📖	Linting Wire with this PR took 18.6 s vs 18.63 s on main (0% faster).
📖	Linting WordPress with this PR took 12.44 s vs 12.49 s on main (0% faster).

Here's an example of your CHANGELOG entry:

* Add combine_flatmap lint rule.  
  [ldct](https://github.com/ldct)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

Generated by 🚫 Danger