switch to ESM and update config/test wiring

[crazy-max]

needed for #960

https://github.com/docker/actions-toolkit/actions/runs/21432262600/job/61714304782?pr=960#step:2:333

#13 [build 1/1] RUN --mount=type=bind,target=.,rw   --mount=type=cache,target=/src/.yarn/cache   --mount=type=cache,target=/src/node_modules   yarn run build && mkdir /out && cp -Rf lib /out/
#13 11.52 node_modules/@actions/github/node_modules/@octokit/request-error/dist-types/types.d.ts(2,46): error TS2304: Cannot find name 'ErrorOptions'.
#13 11.52 src/context.ts(21,25): error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("@actions/github")' call instead.
#13 11.52   To convert this file to an ECMAScript module, change its file extension to '.mts', or add the field `"type": "module"` to '/src/package.json'.
#13 11.52 src/git.ts(23,23): error TS2307: Cannot find module '@actions/github/lib/context' or its corresponding type declarations.
#13 11.52 src/github.ts(31,25): error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("@actions/github")' call instead.
#13 11.52   To convert this file to an ECMAScript module, change its file extension to '.mts', or add the field `"type": "module"` to '/src/package.json'.
#13 11.52 src/github.ts(32,33): error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("@actions/github/lib/utils")' call instead.
#13 11.52   To convert this file to an ECMAScript module, change its file extension to '.mts', or add the field `"type": "module"` to '/src/package.json'.
#13 11.52 src/github.ts(33,23): error TS2307: Cannot find module '@actions/github/lib/context' or its corresponding type declarations.
#13 11.52 src/types/git.ts(17,40): error TS2307: Cannot find module '@actions/github/lib/context' or its corresponding type declarations.
#13 ERROR: process "/bin/sh -c yarn run build && mkdir /out && cp -Rf lib /out/" did not complete successfully: exit code: 2

[crazy-max]

Can't use nodenext yet because of @actions/artifact/lib imports:

actions-toolkit/src/github.ts

Lines 23 to 27 in f64486a

	import {CreateArtifactRequest, FinalizeArtifactRequest, StringValue} from '@actions/artifact/lib/generated';
	import {internalArtifactTwirpClient} from '@actions/artifact/lib/internal/shared/artifact-twirp-client';
	import {isGhes} from '@actions/artifact/lib/internal/shared/config';
	import {getBackendIdsFromToken} from '@actions/artifact/lib/internal/shared/util';
	import {getExpiration} from '@actions/artifact/lib/internal/upload/retention';

Tried to vendor it but then Jest fails so would need to migrate Jest which will be used to transform files into valid JS based on Babel configuration without need of ts-type.

[crazy-max]

This could be easily solved in the future with actions/toolkit#2256