Java FFI functions are incorrectly treated as regular functions

[nipunayf]

Description

ATM, these are rendered under the Functions section in the Artifacts list. As a result, a flow diagram is generated for these functions, which is misleading because a flow cannot be defined for them, and any such edits would cause syntax errors in the code.

Steps to Reproduce

Render the diagram for the following source code.

function execCommand(handle runtime, handle command) returns handle|error = @java:Method {
    'class: "java.lang.Runtime",
    name: "exec",
    paramTypes: ["java.lang.String"]
} external;

Version

1.6.1

Environment Details (with versions)

No response

[KavinduZoysa]

Addressed via ballerina-platform/ballerina-language-server#671