Remove umd builds from all packages

[texodus]

The intention of this PR is to make perspective easier to use from a bundler, especially webpack.

• Remove umd builds from all packages. Importing the root package e.g. @finos/perspective now resolves to an inlined build in ES Module (esm) format, and an asset-parallel build in any plugin-assisted bundler script or in the dist/cdn directory. As a result of this change, it is no longer possible to include pre-built perspective directly in an HTML <script> tag without <script type="module">, which is well supported by all browsers. A bundler and plugin (or other) can still produce such output if desired.
• @finos/perspective-workspace is no longer a supserset of @finos/perspective-viewer, and the latter must now be imported separately (as well as themes).
• Separates C++ emscripten build into node and web bundles. This makes the build much slower, but to do otherwise (e.g. currently) emscripten's JS output contains both JS (URL()) and node (require("path")) code blocks emitted, which drives some bundlers (e.g. webpack) crazy.
• Adds missing tests for the @finos/perspective-cli package.
• Upgrades boost to 1.8.3 and emsdk to 3.1.24.