Allow to customize the insertion location for the code generated by source actions

[testforstephen]

Add a preference "java.codeGeneration.insertionLocation" to control the location where the generated code via source actions is inserted.

Below are allowed values:

• "lastMember" - Insert the generated code as the last member of the target type.
• "beforeCursor" - Insert the generated code before the member where the cursor is located.
• "afterCursor" - Insert the generated code after the member where the cursor is located. This is the default behavior.

And this preference only applies to the source actions below:

• Override/Implement Methods
• Generate Getters and Setters
• Generate hashCode() and equals()
• Generate toString()
• Generate Constructors
• Generate Delegate Methods

[testforstephen]

Here is a demo based on vscode client implementation.

Screen.Recording.2021-04-01.at.16.17.44.mov

[jdneo]

Played around and LGTM.