Update build system for external plugins

[garrettjstevens]

After talking with @rbuels, this PR removes support for previous webpack-built plugins and switches over to using TSDX (ref).

There's a template with information in the README about how to use it here: https://github.com/GMOD/jbrowse-plugin-template. (The template is private for now, would be made public on merge of this)

Also adds a couple notes in the docs about this.

I've got a PR going for switching the GDC plugin to this. I could also put in PRs for other plugins if wanted (https://github.com/cmdcolin/jbrowse-plugin-biothings-api and https://github.com/cmdcolin/jbrowse-plugin-ucsc-api are mentioned in the docs, are there others we should update?).

There's a bug relating to build naming in the current version of TSDX that gets fixed with patch-package (see jaredpalmer/tsdx#669), and when that bug gets fixed we can then update development-tools to not use patch-package and end users shouldn't notice any difference.

[cmdcolin]

removes support for previous webpack-built plugins

What causes this?

[garrettjstevens]

removes support for previous webpack-built plugins

What causes this?

Two things:

1. On webworker, global stuff is on self instead of self.window
2. Removes the webpack config from @jbrowse/development-tools

[codecov]

Codecov Report

Merging #1578 (0313182) into master (69e98e4) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1578      +/-   ##
==========================================
- Coverage   59.51%   59.47%   -0.05%     
==========================================
  Files         435      434       -1     
  Lines       19565    19549      -16     
  Branches     4568     4566       -2     
==========================================
- Hits        11645    11627      -18     
- Misses       7626     7628       +2     
  Partials      294      294              

Impacted Files	Coverage Δ
packages/core/PluginLoader.ts	25.64% <0.00%> (-2.14%)	⬇️
packages/core/ReExports/modules.ts	75.00% <ø> (ø)
products/jbrowse-web/src/rpc.worker.ts	22.03% <0.00%> (ø)

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[rbuels]

this removal of the webpack stuff is a breaking change, but I feel that instability is probably OK because it's early days still