Session export in IGV format doesn't include type tag in resource section

[iromeo]

IGV 2.16.2 also includes type tag. E.g.:

    <Resources>
        <Resource path="f2/foo.bed" type="bed"/>
        <Resource path="f1/foo.bed" type="bed"/>
        <Resource path="https://encode-public.s3.amazonaws.com/2021/05/05/7d7f2892-7ccd-46b3-a073-659555bf7cd3/ENCFF637HNV.bigBed" type="bigbed"/>
        <Resource path="https://encode-public.s3.amazonaws.com/2021/05/05/d7bfbe1f-082e-4455-a05e-354609ae0a5f/ENCFF783FUP.bigWig" type="bigwig"/>
        <Resource path="https://encode-public.s3.amazonaws.com/2021/05/05/fcb738c7-efaa-4b6a-bc4a-7fc2d408bafa/ENCFF789OLP.bed.gz" type="bed"/>
        <Resource path="f1/foo1.bed" type="bed"/>
    </Resources>
    ```
    
At the moment, JBR doesn't save such field into the`Resource` tag, but such sessions could be sucessfully opened in latest IGV 2.16.2

So questions arise:
* how critical is for IGV session to have this field missing is it required by newer sessions or an optional thing
* does legacy IGV versions (that don't serialize the fields) will open session with such tag (e.g. IGV 2.9.0)
* which values does `type` could have
* should we add `type` tag o into our export into IGV format dialog.