refactor(generators): Introduce DartGenerator 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

Part of #7085.

Proposed Changes

Introduce a DartGenerator class to be the class of which dartGenerator 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 dartGenerator to Dart in generators/dart/*.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 DartGenerator class and Order enum should be automatically generated by our JSDoc configs.

Additional Information

It is now possible to create multiple dartGenerator 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{dartGenerator,DartGenerator}from'blockly/dart';constcustomJSGenerator=newDartGenerator();// Optionally copy standard generator functions to new instance:Object.assign(customDartGenerator.forBlock,dartGenerator.forBlock);// Optionally add/replace some/all generator functions:customDartGenerator.forBlock('block_type')=function(block){/* new implementation */};// Optionally change settings:customDartGenerator.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{dartGenerator,Order}from'blockly/dart';// Old:dartGenerator.forBlock('answer_expression_block')=function(block){constcode='42';return[code,dartGenerator.ORDER_ATOMIC]}// New:dartGenerator.forBlock('answer_expression_block')=function(block){constcode='42';return[code,Order.ATOMIC]}

DEPRECATION: The .ORDER_* constants on dartGenerator 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.