Add font formatting support to table cells

[dsmmcken]

As a I would like to have more control over text formatting within a cell. Users should be able to control whether a cell is bold, italic, underlined or strikethrough, and text alignment left, center or right. We will not support mixed rich text, the formatting will apply to the cell as whole only.

• Bold, Italic, Underline and/or Strikethrough the value of a cell through a format_columns expression. API to proposed first.

  # format to be proposed, something along these lines though
  t = t.format_columns("x = Bold(x)")
  

• Set Bold, Italic, Underline and/or Strikethrough conditionally using the table conditional formatting UI
  
  note: icons will be needed for strikethrough and underline.
• Set text alignment (left, center, right) of a cell through a format_columns expression. API to proposed first.
• Set text alignment of a column via a table header context menu option
  

Text alignment should follow similar formatting priority to number formatting. Where: Set in the UI on the column > Set as column expression in the query > The default alignment based on column type.