fix: only consider edges which are present in DOM for flowchart

[ad1992]

Sometime mermaid db getEdges function returns edges which aren't present in the DOM hence I am filtering the edges to consider only which are present in DOM. I have created an issue in mermaid as well for the same, will try to fix this in mermaid library itself.

flowchart TB   
    subgraph conference
        frontend
        backend
        security
    end
    frontend --> |Dive into frontend frameworks| conference
    backend --> |Learn all about backend| conference
    security --> |securing web apps| conference

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Name	Status	Preview	Updated (UTC)
mermaid-to-excalidraw	✅ Ready (Inspect)	Visit Preview	May 13, 2024 7:55am