Add write support for GPOS type 2 lookups (kerning)

[Balearica]

Description

This PR adds write support for GPOS type 2 lookups (kerning).

• For fonts that have a GPOS table, that GPOS table is now written to the output font.
  • Currently only kerning tables are supported--the others are dropped.
• For fonts that do not have a GPOS table, but do have a kern table (so the kerningPairs property is populated), the kerningPairs property is converted into a GPOS table and written to the output.
  • This method can also be used for easily implementing pair kerning in custom fonts.

Motivation and Context

This feature has been requested by many users over the years. See #95 and #547, among others.

How Has This Been Tested?

• Tests implement the examples on the Microsoft typography website, and confirm the results match what is documented there.
  • This mirrors how the tests for gsub write support work.
• A test confirms that a font that contains both format 1 and format 2 type 2 lookups can be saved and parsed without changing the contents of the gpos table.
• A test confirms that data from the kern table is properly converted into a gpos table.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I did npm run test and all tests passed green (including code styling checks).
• I have added tests to cover my changes.
• My change requires a change to the documentation.
• I have updated the README accordingly.
• I have read the Contribute README section.