Environment completion: only insert newline when necessary

This change makes a newline to be inserted after \end{env} only if there was text to the right of the cursor when the user pressed enter. It also makes sure the newline respects the existing indentation. The special case of \[...\] is also handled.

BUG: 177611

Test cases:

In each case below, the pipe character is meant to denote the position of the cursor, not an actual character. Place the cursor as indicated (in insert mode) and press the Enter key on the keyboard.

1. Before pressing Enter:

\begin{equation}|

After pressing Enter:

\begin{equation}
	|
\end{equation}

2. Before pressing Enter:

\begin{align}
	\begin{split}|
\end{align}

After pressing Enter:

\begin{align}
	\begin{split}
		|
	\end{split}
\end{align}

3. Before pressing Enter:

\begin{equation}|abc123

After pressing Enter:

\begin{equation}
	|
\end{equation}
abc123

4. Before pressing Enter:

\begin{align}
	\begin{split}|abc123
\end{align}

After pressing Enter:

\begin{align}
	\begin{split}
		|
	\end{split}
	abc123
\end{align}

5. Before pressing Enter:

\[|abcasdf

After pressing Enter:

\[
	|
\]
abcasdf