Add ley new --ts option for .ts extension + ESM

[karlhorky]

Hey @lukeed 👋 hope you're well.

Quick PR to add a --ts option to ley new to add a .ts extension and use ESM in migrations:

$ ley new todos --ts
#=> migrations/004-todos.ts

$ cat migrations/004-todos.ts
#=> export async function up(client) {
#=> }
#=> 
#=> export async function down(client) {
#=> }

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (0cf486e) 48.82% compared to head (1105a67) 48.82%.

Files	Patch %	Lines
index.js	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #38   +/-   ##
=======================================
  Coverage   48.82%   48.82%           
=======================================
  Files           2        2           
  Lines         213      213           
=======================================
  Hits          104      104           
  Misses        109      109           

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