Add package exports fallback for the CLI, dist files, etc.

[dcodeIO]

With the fallback, legacy cases like

import asc from "assemblyscript/cli/asc";
import options from "assemblyscript/cli/util/options";
import distAsc from "assemblyscript/dist/asc";

including others we haven't though of yet are covered by always resolving ./* -> ./*.js. Fixes #1522

At some point we'll most likely want to break this, either to pull packages out of the main packages or make nicer entry points like just /loader, but this gives us sufficient backwards compatibility (on node >= 15, need to check 14) for now so is good to do :)

Edit: According to the node 14 docs, the fallback should work there as well.

• I've read the contributing guidelines

[SebastianSpeitel]

Maybe a test suite which tries to import/resolve the subpackages/named exports under different conditions (node versions, type=commonjs, type=module, using TS, using different bundlers, ...) could be useful.

Sorry, if https://github.com/AssemblyScript/assemblyscript/tree/master/tests/packages already includes thoses.

[dcodeIO]

I agree that having actual tests would be best, yeah. Needs to somehow install the package before it's published then, via a file: dependency or the like I guess.

[dcodeIO]

Also note that 0.17.0 breaks with node <=12 anyhow (14 entered LTS just recently) due to making the ES module the default, with /umd/index.js being what to use on older node versions now. Hence, for exports syntax, node >=14 is of interest. Going to test with a node 12 manually for now to see how it behaves, so I can give proper recommendations.

[dcodeIO]

Turns out that node 12 has some sort of experimental support with a warning:

(node:4732) ExperimentalWarning: The ESM module loader is experimental.
internal/process/esm_loader.js:90
    internalBinding('errors').triggerUncaughtException(
                              ^

Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './cli/asc' is not defined by "exports" in path\to\node_modules\assemblyscript\package.json imported from path\to\test\esm\index.js

and does not understand the * syntax yet, so to support node 12 this change needs to be

    "./*": "./*.js",
    "./cli/asc": "./cli/asc.js",
    "./cli/util/options": "./cli/util/options.js",
    "./dist/asc": "./dist/asc.js"

with the fallback triggering for node >=14, and being otherwise ignored.

Raises the question whether we should rather intentionally break with node 12, so users upgrade to a version of node that we can guarantee will work for at least a while.

[dcodeIO]

node 12 docs: https://nodejs.org/dist/latest-v12.x/docs/api/esm.html#esm_package_entry_points
node 14 docs: https://nodejs.org/dist/latest-v14.x/docs/api/packages.html#packages_package_entry_points
node 15 docs. https://nodejs.org/dist/latest-v15.x/docs/api/packages.html#packages_package_entry_points (== 14)

[dcodeIO]

For completeness, node 10 breaks on TextDecoder used as a global in the loader btw, and does not understand exports syntax at all, requiring lib/loader/umd otherwise. Probably not worth to support, but then maybe fallbacks for 12, simply because we can, are fine.

[dcodeIO]

Merging this for now since I expect most problems to be solved by it, but I also have an external test package now that I can first verify with and perhaps make a test of eventually.

[dcodeIO]

Test package: https://github.com/dcodeIO/assemblyscript-esm-test/runs/1326736751

Import type	^15	15.0.0	^14	14.0.0	^13	13.0.0	^12	12.0.0	^11	11.0.0	^10	10.0.0
import "pkg"	✔	✔	✔	✔	✔	❌2	✔	❌3	❌3	❌3	❌3	❌3
require("pkg")	✔	✔	✔	✔	✔	❌1	✔	❌4	❌4	❌4	❌4	❌4
require("pkg/umd")	✔	✔	✔	❌1	❌1	❌1	❌1	✔	✔	✔	❌5	❌5

1. Package subpath 'pkg' is not defined by "exports" in package.json
2. SyntaxError: Cannot use import statement outside a module
3. SyntaxError: Unexpected identifier
4. SyntaxError: Unexpected token export
5. ReferenceError: TextDecoder is not defined

TL;DR: Node >=^12 works with the new stuff, 11 works with legacy require, and less than 11 fails due to TextDecoder not being a global in the loader, but otherwise (compiler, rtrace) also works with legacy require.

[jtenner]

So it works with node 14 and 15 just fine? I'm okay with that.