Fix(VIM-2933): Reloading/sourcing .ideavimrc does not initialize new plugins

[chylex]

Found two issues related to VIM-2933:

1. Clicking the Reload button did not call VimExtensionRegistrar.enableDelayedExtensions(), so new extensions were not getting initialized.

   At this point, there are 3 places in the code that follow the pattern of setting the vimscriptExecutor.executingVimscript flag, executing a script, and then initializing new extensions. On the other hand, the :source command does not appear to do this at all, it just executes the script and initializes extensions immediately.

   I don't know why extensions have to be delayed, but it seems like the inconsistency and duplication of this behavior should be refactored. If this behavior exists at all, isn't it always necessary? - In which case the delayed initialization + setting of the flag should perhaps be done directly in Executor.execute, rather than having to remember to do it manually anywhere scripts are being executed.

2. It was possible to edit .ideavimrc and run :source ~/.ideavimrc before the file was actually saved. The Reload button ensures the file is saved first, but :source does not.

   This PR ensures that, before executing any script file, if the script file is currently opened in an editor and dirty, it will be saved first. This also means the Reload button itself doesn't need to save the file anymore.

   As I said earlier, :source does (eagerly) initialize new extensions, so this is a slightly different issue. I'm guessing the author of VIM-2933 edited the file, tried sourcing it, and it did not initialize new extensions just because IntelliJ hadn't saved the file yet, so they assumed the initialization was broken entirely; that's what happened to me while I was trying to reproduce the issue.

   Since this fix affects all script file executions, let me know if you'd like me to separate it into 2 commits and/or PRs.

[AlexPl292]

Hi there! Thank you for this update.
Actually, after reviewing your change, I decided to revamp this area a bit. This started to be a bit outdated after all the changes in the plugin.
Also, I added documentation to explain why we need to initialize plugins after the full execution of .ideavimrc.
And I took your suggestion about setting the flag directly in the "execute" function, thank you for that.

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