Introduce an "import" conditional export for each of the chunk
entrypoints (blockly/core, blockly/blocks, blockly/javascript
etc.), and point these at wrappers created by build_tasks.js
that import the corresponding <chunk>_compressed.js file and
export its named exports.

BREAKING CHANGE:

Importing Blockly via

    import Blockly from 'blockly/core';

(and similarly for the other chunk entrypoints) has worked until
now because most build tools (including Webpack in particular)
fuilfil the request for the default export of a CJS module by
providing the module.exports object, rather than an
explicitly-named default export as they would for an ES module.

Since core/blockly.ts (the notional entrypoint for blockly/core)
does not provide a default export, the wrappers created by this
PR do not either.

Code of the above form will therefore break, and should be updated
to use a wildcard:

    import * as Blockly from 'blockly/core';

Introduce an "import" conditional export for the top-level
package entrypoint (blockly), and point it at a wrappers
created by build_tasks.js that imports the existing index.js
file.

BREAKING CHANGE:

Importing Blockly via

    import Blockly from 'blockly';

has worked until now because most build tools (including Webpack
in particular) fuilfil the request for the default export of a
CJS module by providing the module.exports object, rather than an
explicitly-named default export as they would for an ES module.

Since core/blockly.ts does not provide a default export, the
wrapper created by this PR does not either.

Code of the above form will therefore break, and should be updated
to use a wildcard:

    import * as Blockly from 'blockly';

Introduce an "import" conditional export for each of the
langfile entrypoints (msg/en, msg/fr, etc.),, and point them
at wrappers created by build_tasks.js that import the
existing <lang>.js file.

BREAKING CHANGE:

Importing languages via

    import en from 'blockly/msg/en';

has worked until now because most build tools (including Webpack
in particular) fuilfil the request for the default export of a
CJS module by providing the module.exports object, rather than an
explicitly-named default export as they would for an ES module.

Code of the above form will therefore break, and should be updated
to use a wildcard:

    import * as en from 'blockly/msg/en';

For some reason we had a typings/msg/yue.d.ts that did not
correxpond to any msg/json/yue.json.  Delete it.