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 :warning:

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Additional details and impacted files

```diff @@ 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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