fix(gatsby-theme-blog): drop path.join to support windows

[wardpeet]

Description

creating urls with path.join isn working on osx & linux but sadly not on windows. This PR fixes that. I join by / and replace // or more into one /

before:

after:

[sidharthachatterjee]

Can't we use

const url = require('url');
url.resolve(basePath, dir, node.parent.name)

[wardpeet]

url.resolve has another use case. Check out https://nodejs.org/api/url.html#url_url_resolve_from_to it only works for 2 values

Currenly blocked on getting #15451 shipped

[talves]

@wardpeet I tested the joinPath utility you propose on the slug in onCreateNode function (toPostPath) in my example and it will not solve the issue of an unexpected slug path. Instead of getting a slug with back slashes, which is happening now, that should be forward slashes, it returns the path with double back slashes now.

The issue is a node path.join returns a slug path with back slashes on windows that should be forward slashes in the slug for the browser. There needs to probably be a utility that converts paths to a valid relative slug path instead like below.

  const toSlugPath = node => {
    const { dir } = path.parse(node.relativePath)
    return join.path(basePath, dir, node.name).replace(/\\/g, '/')
  }

Of course, the toNotesPath function is a different issue that probably needs the proposed utility, but I did not look at that one.

[wardpeet]

It seems like joinPath isn't doing what I was told it did. It converts \ into \\ to safely store it to disk.

I have a new PR up #15510 that adds urlResolve

[sidharthachatterjee]

Thank you @wardpeet 🥇