Add snippets for operators and builtins

[EmilyGraceSeville7cf]

[bmalehorn]

Hey @EmilySeville7cfg , thanks for the PR! I don't use snippets much myself, and I didn't know how to set them up.

Overall this looks good, but I'm going to follow it up with some of my own changes that I think will make it easier for others to use / more consistent with other snippets:

• format json
• rename "An if operator" -> "If operator"
• change indentation 2 spaces -> 4 spaces, which is what fish_ident uses
• replace echo with empty string, matching snippets used in typescript
• remove description since most people probably aren't writing descriptions

You can follow the instructions here to save your personal snippets, which will override the ones I'm setting in this extension.