Support goto definition across files

[Marwes]

Goto definition currently only works within a single file, it would be great if it would work across files. Specifically I am thinking of using it to go to standard library functions.

testing.diff
        // ^ Goto jumps to `testing.flux:diff` in the standard library 

The semantic analysis does assign Symbol values which should be unique across files which ought to contain enough information to find the right location However, as the standard library is embedded in the binary we may need to display the source code in a special buffer somehow, or allow users to configure where the standard library is so the LSP can link to it.