fix: table header in markdown export

[fuscodev]

Removing colgroup tags in html string before exporting in markdown

From isHeadingRow() function in @joplin/turndown-plugin-gfm :
// A tr is a heading row if:
// - the parent is a THEAD
// - or if its the first child of the TABLE or the first TBODY (possibly
//   following a blank THEAD)
// - and every cell is a TH

before -> after