Preserve multiple spaces on plain text paste

[zoltanhosszu]

Plain-text paste collapsed runs of consecutive spaces, breaking ASCII tables and other space-aligned content. The collapse comes from HTML/Lexical whitespace normalization. This patch substitutes runs of 2+ spaces with a regular space + &nbsps before Marked sees them, so the spacing survives the round trip while single spaces (and code-block paste) are untouched.

Example table

+--------+--------+--------+
| Name   | Score  | Level  |
+--------+--------+--------+
| Alice  |   87   |   3    |
| Bob    |   42   |   7    |
| Clara  |   65   |   5    |
+--------+--------+--------+

Expectation

Turned into codeblock:

Reality

Turned into codeblock: