Refactor exports for tree-shaking optimization

[tani]

This PR restructures the export pattern to enable better tree-shaking by creating dedicated namespace files:

• Added as/as.ts: Exports type utilities (Optional, Readonly, Unoptional, Unreadonly) as named exports
• Added is/is.ts: Exports all type predicates (Any, Array, ArrayOf, etc.) as named exports
• Updated mod.ts: Now exports both namespace (* as is) and individual exports (*) patterns

Benefits

• Enables tree-shaking when importing specific functions (e.g., import { is } from 'unknownutil')
• Maintains backward compatibility with existing wildcard imports
• Reduces bundle size for consumers who only need specific utilities

Test

$ cat test.ts
import { is } from "./mod.ts";

is.Any(1);
$ deno run -A npm:tsdown ./test.ts
ℹ tsdown v0.14.1 powered by rolldown v1.0.0-beta.32
ℹ entry: test.ts
ℹ Build start
ℹ dist/test.mjs  0.42 kB │ gzip: 0.28 kB
ℹ 1 files, total: 0.42 kB
✔ Build complete in 25ms
$ cat tist/test.mjs
//#region ../is/any.ts
/**
* Assume `x is `any` and always return `true` regardless of the type of `x`.
*
* Use {@linkcode [is/unknown].isUnknown|isUnknown} to assume that a value is `unknown`.
*
* import { is } from "@core/unknownutil";
*
* const a = "a";
* if (is.Any(a)) {
*   const _: any = a;
* }
*/
function isAny(_x) {
        return true;
}

//#endregion
//#region test.ts
isAny(1);

//#endregion
export {  };%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.88%. Comparing base (e377132) to head (2ecdb1a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
- Coverage   99.89%   99.88%   -0.01%     
==========================================
  Files          52       54       +2     
  Lines         954      903      -51     
  Branches      120      120              
==========================================
- Hits          953      902      -51     
  Misses          1        1              

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[lambdalisue]

lgtm!