Add support for upcoming 0.15.0 PureScript release

[JordanMartinez]

When PureScript 0.15.0 comes out, there will be some significant changes:

• CommonJS FFI files will no longer compile as one must use ES modules
• The purs bundle command is being removed in favor of bundlers like esbuild and webpack
• The browser backend for the REPL will be removed since it depends on purs bundle

However, pulp should continue to work regardless of whether one is using PS 0.14.x or PS 0.15.x. Thus, supporting v0.15.0 means the following if the PureScript version being used is >= v0.15.0:

• dropping support for pulp server when purs bundle is called implicitly (e.g. pulp server -t app.js or pulp server --optimize)
• dropping support for pulp browserify, which seems to only work on CommonJS modules
• dropping support for pulp build when purs bundle is called implicitly (e.g. pulp build -t app.js or pulp build --optimize)

Rather than running those commands, the error should direct users to the v0.15.0 Migration Guide.

Although the REPL is being modified, pulp doesn't do anything special for the --port argument, which is what triggers the browser backend for the REPL.