Adopt type=module

[Fil]

follow changes in d3-format:

• type=module
• add exports
• remove zip
• license: ISC
• update dependencies

TODO:

• [x] update README.md
• [x] depends on d3-array@3, d3-geo@3, d3-tricontour@(n+1)
• [x] switch the default branch to main

[martinfrances107]

I have inspected the diff ... All looks good - nothing I can see that raises alarms.

With such a large change to the plumbing... as a third party double check I can report

"The transition to the new environment is smooth all the code for me works as before."

I have been frequently running the old version on the test over the last few weeks.. and my feeling is things have gotten much faster ( with the move to Mocha)

As someone who has been generating .. lots of PR's with minor things I thought making a PR about all the variables which could be defined as const...too small to report - but I am glad to see those things elevated to the modern standards.

Last part of my review... all the text in the README.md reads smoothly

TLGTM

[Fil]

I'll gladly merge a change for let and const