Remove Internet Explorer support

[rachel-fenichel]

Is your feature request related to a problem? Please describe.

On June 15, 2022 Microsoft announced that "Internet Explorer 11 has retired and is officially out of support."

Supporting IE takes a decent amount of work and requires workarounds in both implementations and the build system.

Describe the solution you'd like

• Remove uses of userAgent.IE in Blockly core.
• Remove isIe and IE from useragent.ts
• Remove workaround in flyout_base
• Remove IE from test matrix (internal documentation)
• Remove IE from PR template
• Remove workarounds for IE in bootstrap.js
• Remove IE workarounds from stripped-down base.js
• Update generated target to ES6/es2015 #6328

Describe alternatives you've considered

Continue supporting IE until the heat death of the universe or 2029 (whichever comes sooner).

Additional context

This is a breaking change, although hopefully for very few end users.

[rachel-fenichel]

Other places where we have workarounds for IE:

• workspaceSvg.setBrowserFocus
• css.ts: alternatives to cursor: grabbing;
• xml.ts: blockToDom
• grid.ts: update

[rachel-fenichel]

• svg_math.ts: is3dSupported looks like it was mostly a workaround for IE not supporting CSS transforms on SVG elements. The work here is to check whether the Firefox problem still repros and delete as much of this function as possible--meaning ideally collapse it down to true and then delete it all.

[rachel-fenichel]

• flyout_base.ts: positionAt_ uses setAttribute on the SVG element for IE and Edge, which is no longer necessary.

[rachel-fenichel]

• utils/style.ts: getStyle, getComputedStyle, and getCascadedStyle