fix(snipmate): correctly escape backtick

[danilshvalov]

Let's have the following snippet:

snippet src
	\`\`\`$1
	$0
	\`\`\`

Without this patch, it will expand into:

\`\`\`

\`\`\`

After this fix, it will expand into what is expected:

```

```

[L3MON4D3]

Thank you for making a PR for this :)

Are you up for adding a test to ensure this doesn't regress?
You should be able to run current test via just make test, and can append a new test for this in tests/integration/parser_spec.lua

[L3MON4D3]

Nice :)
One more test for the edge-case I mentioned? (\\\` inside regular backticks)
(And could you squash the commits after?)

[danilshvalov]

Nice :) One more test for the edge-case I mentioned? (\\\` inside regular backticks) (And could you squash the commits after?)

Done

[L3MON4D3]

Yup, ty!