refactor(generators): Introduce JavascriptGenerator class, Order enum

[cpcallen]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

First part of #7085.

Proposed Changes

• Introduce a JavascriptGenerator class to be the class of which javascriptGenerator is an instance (without any additional properties making it a singleton).
• Introduce an Order enum (for now Closure-style) for the operator precedence table.
• Don't rename javascriptGenerator to JavaScript in generators/javascript/*.js.

Behaviour Before/After Change

There should be no changes in behaviour.

Reason for Changes

See #7085.

Test Coverage

Passes npm test; no changes to manual testing anticipated.

Documentation

Documentation for the JavascriptGenerator class and Order enum should be automatically generated by our JSDoc configs (though if this is not currently the case then we should fix that).

Additional Information

It is now possible to create multiple JavascriptGenerator objects, which can share (or not share) block generator implementations. Note that new instances do not by default come with any generator functions pre-installed.

import {javascriptGenerator, JavascriptGenerator} from 'blockly/javascript';

const customJSGenerator = new JavascriptGenerator();

// Optionally copy standard generator functions to new instance:
Object.assign(customJSGenerator.forBlock, javascriptGenerator.forBlock);

// Optionally add/replace some/all generator functions:
customJSGenerator.forBlock('block_type') = function(block) {/* new implementation */};

// Optionally change settings:
customJSGenerator.INDENT = '\t';  // Use tabs to indent generated code.

We now provide an enum of the operator precedence values (order of operations) constants as a separate export:

import {javascriptGenerator, Order} from 'blockly/javascript';

// Old:
javascriptGenerator.forBlock('answer_expression_block') = function(block) {
  const code = '42';
  return [code, javascriptGenerator.ORDER_ATOMIC]
}

// New:
javascriptGenerator.forBlock('answer_expression_block') = function(block) {
  const code = '42';
  return [code, Order.ATOMIC]
}

DEPRECATION: The .ORDER_* constants on javascriptGenerator are deprecated; they still exist but are no longer part of the documented API and will be removed in a future version of Blockly. We recommend that you update your custom generator code to use the Order enum at your earliest convenience.

[cpcallen]

Oh, bother: I merged before pushing commit with comment update. Drat.