Spreadsheet+LibSyntax: Never insert spans directly

Function `CellSyntaxHighlighter::rehighlight()` direct inserted spans
to TextDocument `m_span` vector missing out important reordering and
merging operation carried out by `TextDocument::set_spans()`.

This caused overlapping spans for a cell with only a `=` symbol
(one for the actual token and one for the highlighting) to
miscalculate `start` and `end` value and a `length` value (of
`size_t` type) with a `0-1` substraction (result: 18446744073709551615)
causing `Utf32View::substring_view()` to fail the
`!Checked<size_t>::addition_would_overflow(offset, length)` assertion

This remove the possibility to directly alter `TextDocument`'s spans
thus forcing the utilization of `HighlighterClient::do_set_spans()`
interface function.

Proper refactor have been applied to
`CellSyntaxHighlighter::rehighlight()` function

Userland/Libraries/LibSyntax/Highlighter.h

@@ -125,7 +125,6 @@ public:
     }
 
 private:
-    virtual Vector<GUI::TextDocumentSpan>& spans() override { return m_spans; }
     virtual Vector<GUI::TextDocumentSpan> const& spans() const override { return m_spans; }
     virtual void set_span_at_index(size_t index, GUI::TextDocumentSpan span) override { m_spans.at(index) = move(span); }