Escape invalid characters in file names when generating bridging code

[marcprux]

I have a NativeViewModel+Bridging.swift file with the code:

public var swiftClosure1Var: (Int) -> String = { i in "value = \(i)" }

in a package with skip.yml:

skip:
  mode: 'swift'
  bridging: true

It raises the following compile error in the child SKIP_BRIDGE build because it is tying to generate a name with the "+" in it. We should escape out invalid characters from the file name (like maybe replacing it with the codepoint like JNI signatures, or just an underscore).

/Users/marc/Library/Developer/Xcode/DerivedData/Skip-Everything-aqywrhrzhkbvfseiqgxuufbdwdft/SourcePackages/plugins/skipapp-appdroid.output/AppDroidModelTests/skipstone/AppDroidModel/.build/AppDroidModel/swift/plugins/outputs/swift/AppDroidModel/destination/skipstone/SkipBridgeGenerated/NativeViewModel+Bridging_Bridge.swift:32:21: error: expected '(' in argument list of function declaration
30 | }
31 | @_cdecl("Java_app_droid_model_NativeViewModel+BridgingKt_Swift_1swiftClosure1Var")
32 | func NativeViewModel+BridgingKt_Swift_swiftClosure1Var(_ Java_env: JNIEnvPointer, _ Java_target: JavaObjectPointer) -> JavaObjectPointer {
   |                     `- error: expected '(' in argument list of function declaration
33 |     return SwiftClosure1.javaObject(for: swiftClosure1Var)!
34 | }