On Windows you should find your file in the following location:
On Linux you should find your save file in this location (thanks to Scott for the info):
Make sure to make a backup copy of this file if you plan on editing your save.

Make sure your game is closed before editing this file!

Despite having a "dat" extension, the contents of the file are plaintext rather than binary data. Open this file in your text editor of choice. I recommend VS Code/VS Codium. The file is really just a JSON file, though your editor might not detect it as such right away. You can change the working language in VS Code with the shortcut "Ctrl+K M", and selecting JSON. After this, you can choose to format your document, so it's easier to work with it. The end result should look something like this:
At this point, you can change any values you want around to edit your save. While I don't think it's strictly necessary, I recommend minifying the JSON before saving the file.

Editing your save is definitely considered cheating! If you're okay with this, read on.

The absolutely most annoying part of going for 100% in this game is grinding for coins in order to buy all gallery items and hats. You can easily spend twice the time (or more) you've already spent completing everything else just grinding away. Thankfully, you can edit your save to give yourself as many coins as you want! This will save you a ton of time on what many consider a pointless grind.

Open your save file following the previous section. You'll find your coin count inside a very, very long string in the save file. Look for the data under "StringData". You'll find some key-value pairs; find the one of the save slot you want to edit. In my case it was the one with "Key": "Save_Slot1". You'll find the really long string in the part that says "Value". Your coins are saved in a part that says "coins;123\n" (however many coins you had). Just change this number to a ludicrous amount of coins and save your file as per the previous section.

---

And you're done! Enjoy not grinding for hours!

Having a rough time with a certain level? You can edit your save to set levels as completed, allowing you to skip those levels! While setting a specific level as completed is possible, the easiest way to do this is to set all levels as completed, thus allowing you to freely select and skip any level.

Open your save file following the save editing section. You'll find the level clear information inside a very, very long string in the save file. Look for the data under "StringData". You'll find some key-value pairs; find the one of the save slot you want to edit. You should see the really long string in the part that says "Value". The level information is stored after the starting bit that should look something like "hasSeenIntro;1\nlastLevel;Level1\ncoins;10\nhat;-1\n". It should start with information on Level 1 and continue all the way up to nearly the end of the string.
This information contains all the clear data for each level, including your best time and which collectibles you got for each level. Just for demonstration purposes (aka you don't have to do this) I formatted it a bit more neatly so you can see this data.
As you can see, every level that is not clear has the same information, which is a bunch of zeroes. You also might have noticed that the save doesn't store the level data in the order of the levels, which makes skipping a particular level a bit difficult without having a good idea of which level is which on the level select. This means editing all levels at once (and thus unlocking/clearing them all) is a lot easier! All you have to do is replace every instance of ";0;0;0;0;0;0;0;0;0;" with ";0;0;1;0;0;0;0;0;0;" and every level will be set as completed, allowing you to skip any level you want! Most text editors should have a Search & Replace tool in order to get it done in an instant. In case you have multiple save files, be careful which one you are changing! If you are not careful you might make changes to multiple save files at once.
And that's it! Feel free to explore the whole game to your heart's content. You can also edit the rest of the numbers to change your unlocks and best times, but I'll leave it at marking levels as completed for now.