Rename enum Blockly.connectionTypes to follow the style guide

[moniika]

Issue

The enum Blockly.connectionTypes does not follow the style guide of using UpperCamelCase and also of generally being a singular noun.

Suggested Fix

Renaming Blockly.connectionTypes to Blockly.ConnectionType for consistency.

Additional context

https://google.github.io/styleguide/jsguide.html#naming-enum-names

[himanshujaidka]

@moniika pls assign this issue to me

[cpcallen]

Closed by #5407.