Go to definition improvements

[cgtudor]

This is built on top of #77 but I can't set that as the base branch on this repo.

This PR accomplishes the following:

• Go to definition (for functions) now works even if the function doesn't have a forward declaration (as long as the file it lives in is included, of course).
• If a function has both a forward declaration and a declaration with a body, go to definition now jumps to the function with a body instead of the forward declaration. If there is only a declaration without body (like nwscript.nss functions) it will jump to them still.
  • This is set as default behaviour but can be turned off (returning to the previous behaviour) in the vscode user settings with "nwscript-ee-lsp.definition": { "preferImplementation": false }