Support a "Convention" concept for the Webpack TogglePointInjection Plugin

[TomStrepsil]

Currently, the configuration of the webpack plugin is fiddly, and has coupled concepts. e.g.

• The variantGlobs and the joinPointResolver need to work with parity of finding a variation, and then "undoing" it to find a control module.
• The toggleHandler is similarly coupled to the variantGlobs, since it unpicks the matched paths.

We could consider bundling all of this complexity into pre-built "Conventions", that take simplified parameters.

e.g. a PathSegmentConvention, that could take parameters:

• triggerPath (or similar)
  • an optional path that is part of the variant match, but not fed into the featuresMap produced. e.g. __variants__ in the default example, is the "trigger path"
  • in other use-cases (see examples), there is no requirement of such a "triggerPath", because all paths are potential candidates for the convention.
• pathSegmentCount (or similar)
  • the number of path segments that make up the convention
    • the default is /<feature>/<variant>, but other examples in the repo include only a single path (e.g. /language/), and there may be use-cases for more
• basePath
  • the base path to construct the full variantGlob e.g. /src/components/ would result in /src/components/**/__variants__/... etc.
• maxDepth
  • how "deep" to look from the basePath - e.g. 1 would be a single path segment /*/ in the resultant variantGlob passed to the plugin, and Infinity perhaps could be a default, which implies a /**/ globstar.