refactor(generators)!: CodeGenerator per-block-type generator function dictionary

[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

Fixes#7084

Proposed Changes

Implements per-block-type generator function dictionary, per proposal #7084.

Behaviour Before Change

CodeGenerator.prototype.blockToCode uses generator function at this[block_type].

Behaviour After Change

CodeGenerator.prototype.blockToCode preferentially uses generator function at this.forBlock[block_type], falling back to this[block_type] if the former does not exist (and issuing a deprecation warning if the latter does).

Code in generators/*/* is updated to install generator functions at the new location.

Reason for Changes

See #7084.

Test Coverage

No changes to manual test procedures anticipated.

Documentation

We should update our generator demos/codelabs to use the new dictionary.

Additional Information

DEPRECATION: as of this PR, adding per-block generator function directly to CodeGenerator instances is deprecated; they should instead be added to the generator's .forBlock dictionary—i.e., existing code of the form:

langGenerator['custom_block_type']=function(block){/* ... */};

should be updated to:

langGenerator.forBlock['custom_block_type']=function(block){/* ... */};

although the former style will continue to be supported at least until until Blockly v11.

BREAKING CHANGE: this PR prioritises the new .forBlock dictionary when doing lookups, and moves the generator functions we provide from their previous location directly on the CodeGenerator instances to the new .forBlock dictionary on each instance.

Most existing custom generator code should continue to work unchanged for the time being, but any code that references the generator functions we provide—e.g. to replace an implementation we provide or to reuse the implementation for a different block type—will need to be updated:

langGenerator['controls_if']=function(block){/* new implementation overriding existing one */};langGenerator['custom_block_type']=langGenerator['existing_block_type'];

must be updated to:

langGenerator.forBlock['controls_if']=function(block){/* new implementation overriding existing one */};langGenerator.forBlock['custom_block_type']=langGenerator.forBlock['existing_block_type'];

[cpcallen]

Investigating test failure, which reproduces locally. I am slightly confused because it is caused by the change to tests/mocha/generator_test.js—a change which should have resolved any related errors.

[cpcallen]

Turns out test in generator_test.js wasn't cleaning up after itself, and was causing failures in insertion_marker_test.js—this is exactly the kind of issue that makes this PR breaking even though the backwards compatibility code should ensure that developers' existing generator code will in most cases continue to work fine.

[cpcallen]

Force-push (and title edit) to make commits conventional.