[DRAFT] feat(gatsby-link): force trailing slash option

[gillkyle]

Description

I've been exploring a commonly requested addition to the core framework, adding a config option to force trailing slashes. This PR adds trailingSlash as an option in gatsby-config, that when set to true will enforce a / being appended to any path or page if it hasn't already been added.

This touches several places (and has some that still have to be done):

• the gatsby-config schema
• gatsby-link and it's helper functions
• gatsby core and likely somewhere like the createPages action to write pages to the cache with the enforced option

Each of these places should have some more tests added to verify that all possible paths are still constructed correctly: /test, /test/, /test#hash, /test/#hash, /test?search=asdf, etc

Documentation

• include an addition in the Gatsby Config API doc

Example

This example clip has trailingSlash: true, and shows Links without trailing slashes being SSR'd with a trailing slash, as well as navigating to the proper path with a trailing slash.

CleanShot.2021-03-12.at.15.03.23.mp4

Related Issues

Related to #6365
Related to #3402

[LekoArts]

I think trailingSlash: false should also enforce non-trailing slash paths as https://www.gatsbyjs.com/plugins/gatsby-plugin-remove-trailing-slashes/ tries to do.

[pieh]

There are more than just trailing slashes options for customization like that - folks might want to end with .html - like reactjs.org does - https://reactjs.org/docs/react-dom.html

Similarly same page can be constructed by using any of these paths: /foo, /foo/, /foo/index.html so if we want to bake path normalization in - we probably should have config that select one of quite few options and not just trailingSlashes: false:

• withTrailingSlashes (/foo/)
• noTrailingSlashes (/foo)
• html suffix (/foo.html)
• index.html suffix (/foo/index.html)

And we don't have to support them from start, but config setting should allow us to add more modes without having to add more fields