Implement support for multi-line rules in Nano Highlighter and Linter

[jspenguin2017]

Highlighting is quite tricky since this is a valid rule:

@ \
    @ \
    | \
    |example.com \
    ^

uBO uses JavaScript for tokenization, but Ace uses RegExp, which makes this difficult.

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Beside highlighting, some linting (missing space before slash, missing four spaces on the next line, etc.) would also be useful.

[jspenguin2017]

@KonoromiHimaries What do you mean? Can you elaborate?