When a step in the `minify` array does not return a `code` string, keep
the last-known-good code (or original input) and feed it to the next
step, rather than breaking out of the loop. Errors and warnings from
the skipped step are still surfaced. If no step in the chain ever
produces code, the existing "Minimizer doesn't return result" error
still fires.

Matches the behavior in html-minimizer-webpack-plugin.

Moves the previously-private TerserPlugin.getEcmaVersion static method
into src/utils.js (where the minimizer functions live), since the
ecma-version derivation only matters for the minimizers that consume
ecma (terserMinify, swcMinify). The plugin now imports getEcmaVersion
from utils.js. No behavior change.

Some minimizers legitimately return only warnings, errors or extracted
comments without producing new code. The plugin previously pushed a
spurious "Minimizer doesn't return result" error in that case, on top
of any real diagnostics the minimizer emitted.

Now:
* no synthetic error is added when `code` is `undefined`
* warnings and errors from the minimizer are still surfaced
* the LICENSE file is still emitted when `extractedComments` are
  returned without code; the banner is only prepended when a new
  source replaces the asset
* the main asset is left unchanged when no minimizer in the chain
  produces code

Map more of webpack's `output.environment` flags to ECMAScript versions
and pick the highest matching version when several flags are set:

* ES2015 now also includes `methodShorthand` and `templateLiteral`
* ES2017 added — triggered by `asyncFunction`
* ES2020 now also includes `dynamicImportInWorker`, `globalThis`
  and `optionalChaining`

Previously the function checked ES2015 before ES2020, so an environment
that supported both (e.g. webpack production defaults) was downgraded to
ES2015 — which left ES2017–ES2020 minification opportunities on the
table. Now the highest version wins.

Effect: when webpack signals modern features, swcMinify can now drop
the unused `catch (e)` binding (ES2019 optional catch binding), which
shows up in the updated snapshots.