Add LiveScript syntax + Svelte with LiveScript panic regression test

[Enselic]

Not only do we get support for a new syntax, we also make bat not panic in Svelte files with embedded LiveScript code (for which this PR also contains a regression test). See #1912 (and #915) for more context.

Adding this syntax also means less "unresolved context reference" warnings when building assets. Now these are the only ones that remain:

- Syntax 'Svelte' with scope 'text.html.svelte' has unresolved context reference ByScope { scope: <source.postcss>, sub_context: None }
- Syntax 'Vue Component' with scope 'text.html.vue' has unresolved context reference ByScope { scope: <source.postcss>, sub_context: None }
- Syntax 'Vue Component' with scope 'text.html.vue' has unresolved context reference ByScope { scope: <source.sss>, sub_context: None }
- Syntax 'Vue Component' with scope 'text.html.vue' has unresolved context reference ByScope { scope: <text.jade>, sub_context: None }
- Syntax 'Vue Component' with scope 'text.html.vue' has unresolved context reference ByScope { scope: <text.pug>, sub_context: None }
- Syntax 'Vue Component' with scope 'text.html.vue' has unresolved context reference ByScope { scope: <text.slm>, sub_context: None }

The added syntax has 10k downloads on Package Control: https://packagecontrol.io/packages/LiveScript

[sharkdp]

Thank you!