Update generated target to `ES6/es2015`

[rachel-fenichel]

As of August 2022, caniuse.com shows that an overwhelming majority of browsers support all parts of ES6 (2015) except for tail call optimization.

This means that transpiling to ES5 and/or polyfilling are unnecessary. It also means we don't have to worry about downlevel iteration from the typescript compiler

To change:

• Update language_out at build_tasks.js:610 to ECMASCRIPT_2015

[rachel-fenichel]

Note that the two outliers are internet explorer and Firefox < 43. It will still be possible to target these with an additional transpilation pass in the application that uses Blockly, but will no longer work out of the box.