Update TypeScript compilation target

[gimenete]

I'm not 100% sure that ES2020 is the best target, but I think the target should be updated because currently the generated JS is too verbose. It even assumes that async/await are not supported by the runtime.

Specially since this library is meant to be used only in cloudflare workers:

• most modern JS features should be supported.
• the size of the generated JS matters.

Here's a list of files with the before/after size:

dist/cjs/exceptions.js  1.8K -> 653B
dist/cjs/index.js       816B -> 816B
dist/cjs/openapi.js     5.9K -> 4.1K
dist/cjs/parameters.js   17K ->  13K
dist/cjs/route.js       8.7K -> 3.6K
dist/cjs/types.js        77B ->  77B
dist/cjs/ui.js          1.7K -> 1.6K
dist/esm/exceptions.js  1.6K -> 364B
dist/esm/index.js       137B -> 137B
dist/esm/openapi.js     5.7K -> 3.9K
dist/esm/parameters.js   16K ->  12K
dist/esm/route.js       8.5K -> 3.4K
dist/esm/types.js        11B ->  11B
dist/esm/ui.js          1.6K -> 1.5K