Add parse-table to Handsontable.helpers

[wojciechczerniak]

Description

We need to make parse-table available externally as one of Handsontable.helpers.

Add support for:

• Parse THEAD to Handsontable settings.colHeaders and back to THEAD
• Parse TR > TH to Handsontable settings.rowHeaders and back to TR > TH
• Import / Export merged cells
• Fixed rows top/bottom
• Nested headers
• Add carriage copy function #5826: Add carriage copy function
• Copy-paste to excel break text to multiple rows #5622: Copy-paste to excel break text to multiple rows
• Carriage return inserted inside string when pasting from Excel, but not from text #5647: Carriage return inserted inside string when pasting from Excel, but not from text
• Cells with longer text are misleading copy&paste #5925: Parse mso-ignore:colspan (check rowspan)
• [6.1.0+] We can't paste merged data from Excel #5940: [6.1.0+] We can't paste merged data from Excel
• Paste multiline cells from googlesheet lost CRLF #5970: Paste multiline cells from googlesheet lost CRLF

API:

• Handsontable.fromHTML()
• Handsontable.fromString()
• hot.toHTML();
• hot.toString();