Upgrade to Pandoc 2.10

[nokome]

Upgrade to Pandoc 2.10 (currently using 2.9.2). Requires upgrades to Pandoc types which according to https://pandoc.org/releases.html#pandoc-2.10-2020-06-29 adds:

• A native Underline constructor for Inline (#6277, Vaibhav Sagar).
• More expressive types for tables (#1024, Christian Despres). Tables can now take attributes; and rowspans and colspans, column headers, multiple row headers, table head and foot can all be represented. (Note, however, that reader and writer support for these features is still lacking, so most users won’t see any differences in table conversion yet. These changes just lay the foundation for further improvements.)

Most if not all of this is currently supported by Stencila Schema but needs to be represented in https://github.com/stencila/encoda/blob/master/src/codecs/pandoc/types.ts