Add more Unicode shortcuts

### Issue

Is there a Unicode symbol you need to type often but can't access easily from Gboard?

If it doesn't already exist in the shortcuts dictionary, you can create an issue for it or add it yourself.

### Notes about formatting

Gboard requires a very specific format for shortcut files to work.


Each line of the file containing a shortcut must have: the shortcut (ex. `\sum`), a single **tab** (` `), the symbol (ex. `∑`), and another a single tab (` `) **at the end of the line.**

Note: Some editors automatically convert tabs to spaces, you can try to avoid this by copy-pasting the tabs from a different line to ensure the spacing is correct.

### Shortcut guidelines

Shortcuts must be the same as the command from LaTeX or resemble the LaTeX shortcuts. If typing the symbol is complex in LaTeX, a shorter shortcut may be used as long as it is in the style of LaTeX commands.

It is at the discretion of the project maintainer to decide whether a certain shortcut should be accepted or not.

### Contributing

* [How to Make a Pull Request with GitHub Desktop](https://youtu.be/mMSOIkkSIag?t=145)

* You can test the file after making changes by making a compressed folder (zip) containing the txt file in a folder.

Locate the shortcuts dictionary TXT file mentioned in the issue and inspect existing LaTeX-style entries. Add an accepted Unicode shortcut using the required tab-separated format, then create the described ZIP file and test it in Gboard. Done means the new shortcut imports and produces the intended symbol.

Written by the indexing model from the issue text.