[FEATURE] Pressing "Enter" in an inline math block should insert a newline after the math block instead of inside it

[ByThePowerOfScience]

Description of the Problem

This is more of a note-to-self than an actual issue since it's fairly trivial to implement, so I'm just making this so I remember to submit a PR adding the functionality myself.

But basically, a newline inside an inline math block (i.e. single-dollar signs: $ ... $) is never valid syntax, so there's no reason not to override the current ENTER-press functionality of "inserting a newline in the middle of the inline math block".

Description of the Solution

There are a few options for what pressing ENTER should do in normal usage:

1. Place a newline at the end of the line and move the cursor to the newly-created next line,
2. Convert the inline math block to a multiline math block, or
3. Same as number 1 but start a new inline math block on the newly-created line.

I propose that we do this:

• ENTER: number 1.
• SHIFT+ENTER: number 3.
• (CTRL/CMD)+SHIFT+ENTER: number 2.
• And if we are inside of a begin ... end block:
  • Pressing ENTER escapes the math block (number 1),
  • Pressing SHIFT+ENTER inserts \\ (for inline matrices such as column vectors), and
  • Pressing (CTRL/CMD)-SHIFT-ENTER converts it to a multiline math block. (number 2)