Change required for Blockly 11.2.0

[MikeJDeck]

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Description

I found that the plugin will fail in Blockly 11.2.0.
I tested the following fix in a local branch.
Replace the line:
if (!Blockly.ContextMenuRegistry.registry.registry_.workspaceSelectAll) {
with
if (!Blockly.ContextMenuRegistry.registry.getItem('workspaceSelectAll')) {

That was the line throwing the error.
I tested this fix in Blockly 11.0.0, 11.1.1, and 11.2.0

Reproduction steps

1. Try the plugin with Blockly 11.2.0
2. Note the error.

Stack trace

No response

Screenshots

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Browsers

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[HollowMan6]

Thank you for pointing this out! This is supposed to be updated already when we support Blockly v11 but we forgot to do so in #62!