Allow newlines and tabs as whitespace (#8)

[phrrngtn]

Fixes #8.

Multi-line formulas (e.g. entered with Alt+Enter) contain \n, \r, and \t characters that the tokenizer previously rejected with an "Unexpected token" error.

Change

• Add an _is_whitespace() helper recognizing space, \t, \n, and \r, and use it in place of the single-space check in the tokenizer's whitespace-trimming loop.
• Because whitespace-vs-intersection is decided later in _fix_whitespace_tokens purely from neighbor token types, newlines are handled identically to spaces: dropped when insignificant, promoted to an intersection operator when between two operands.

Tests

• =SUM(\n\t1,\n\t2\n) tokenizes the same as its single-line form.
• A newline between two ranges (=A1:A10\nB1:B10) becomes an intersection operator.

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[tonyroberts]

@phrrngtn Thanks for the PR!