Expose `FilePath` on `MappingLocation` to let Razor editor know mapping origination.

[NTaylorMullen]

Today the Razor editor has to jump through hoops to determine if a projected mapping location originated from the current file or from a _ViewImports.cshtml. Due to their roundabout way of determining a mappings original location it can occasionally be wrong. We should expose what file created the mapping location so tooling can respond accordingly.

/cc @pranavkm @ToddGrun

[Eilon]

Do we need to have a matching tooling bug to consume this new property?

[NTaylorMullen]

@ToddGrun already has one 😄

[Eilon]

Cool.