Feature Request: Support LSP features for construct names

[oxcrow]

Fortran code becomes verbose, thus using construct names on do loops, blocks, etc. is common way to keep track.

Although they're so widely used in Fortran, they're not supported by fortls.

I think it would be helpful if we could:

• High priority: Jump to the definition of a construct name
• Medium priority: Rename the construct name
• Low priority: Autocomplete the identifier of the construct name

The construct names / labels, are highlighted as blue in this image.

Thanks