Fix #14, upgrade IntelliJ Gradle Plugin and Gradle, fix greedy strings

[mvarendorff]

This PR addresses two issues, upgrades a few dependencies and adds a small note for contributing:

Changes

• Fix for #14 by allowing .IDENTIFIER after an inlineAttribute
• Fix greedy strings using this regex (currently multiple strings on the same line are seen as one because the match extends to the closing quote)
• Upgrade Gradle to 6.1.1 (following a note in the IntelliJ guide on developing plugins with Gradle that mentions a bug causing heap overflows when downloading IntelliJ for development, that also occurred on my machine)
• Upgrade the IntelliJ Gradle Plugin to 0.7.3

Remarks

• I already added changelog entries and bumped the version since I wasn't sure whether there would be more changes to follow; if required, I can remove them however!
• I am not sure what's up with all the added @overrides in the generated files causing all of the single line additions; I guess Grammar-Kit knows what it's doing?
• I am not 100% sure about the README.md change; just when writing this PR I realized it might show up on the JetBrains plugin page? I could move it to CONTRIBUTING.md, somewhere else or remove it, if needed. I just figured it might be nice to have some more information on how to get going when making changes to the plugin somewhere.

[mvarendorff]

@gabrielcolson Ping, just to make sure this didn't get missed (GitHub fails to notify me of things sometimes) :)

[gabrielcolson]

Hey @geisterfurz007

Thanks for the reminder! I have been kind of overwhelmed by work and life these few weeks but I have already read your code and it looks great! I didn't find the time to properly test it yet but I will try to do it today.

Thanks a lot for your contribution!

[mvarendorff]

Perfect, thanks for the fast response! No pressure, I just wanted to make sure it didn't slip through :)

[gabrielcolson]

Hey @geisterfurz007 !

Looks good to me! I am merging this and publishing the new version now. Thanks for your work :)