Disable control sequences

In practice, disabling control sequences means that typing `\` is interpreted as a literal `\` instead of the start of a control sequence.

From the new docs:

- `MathQuill.disableControlSequences()` affects what happens when a user types a `\` character. By default, a backslash is treated as the start of a control sequence. Disabling control sequences causes mathquill to typeset `\` as a literal backslash.
- `MathQuill.enableControlSequences()` reverts the effects of `MathQuill.disableControlSequences()`.