Add continue statement

[GlaireDaggers]

• Adds a new continue token & statement. A continue statement immediately skips to the next iteration of a loop by jumping back to loop top.
• Added tests for the continue statement.

[mhermier]

If you know how to do it, for readability, refactor your changes, introducing one feature at a time with test cases, and prune unnecessary changes.

[ruby0x1]

Thanks @KillaMaaki I'll look closer soon.

It's fine without refactoring since we're going to squash it into one commit anyway, it's a single feature.

@KillaMaaki If you had already explored do while, you're welcome to make a PR so we can see the scope of the change.

Thanks for the PR!

[avivbeeri]

This PR seems fine, tests all pass. Needs documentation though:

• Mention continue in the syntax->reserved words section
• Add a section for continue on the Control Flow page

[ChayimFriedman2]

I've never felt the need in continue.

[avivbeeri]

The presence of continue was previously discussed, and requested in here: #817

[ruby0x1]

I've been testing this in my engine for a while and haven't seen any issues or surprises, and is great to have.