Add support for tectonic -X build

[figitaki]

When trying to setup texlab with tectonic's -X build functionality, I ran into an issue where forward sync was not able to find the output pdf. This occurs because tectonic outputs to $projectRoot/build/default/default.pdf. This change adds a configuration option for build.outputFilename which will override the derived pdf_path if present. This allows the for the following config (assuming Skim as the previewer)

{
  "texlab.auxDirectory": "build/default/",
  "texlab.build.filename": "default.pdf",

  "texlab.build.executable": "tectonic",
  "texlab.build.args": [
    "-X",
    "build",
    "--keep-logs",
    "--keep-intermediates"
  ],
  "texlab.build.onSave": true,
  "texlab.build.forwardSearchAfter": true,

  "texlab.forwardSearch.executable": "/Applications/Skim.app/Contents/SharedSupport/displayline",
  "texlab.forwardSearch.args": ["-g", "%l", "%p", "%f"]
}

[pfoerster]

Thanks for the PR!

[figitaki]

Glad to contribute! Pushed some updates, lmk if that looks better. Can submit an update to the wiki if this lands too.

[pfoerster]

Can submit an update to the wiki if this lands too.

Sure, looking forward! Thanks again for the PR.