Possible extensions system for 1.0.0

[jmhobbs]

With the rewrite, I'm looking to support #10.

This is roughly the proposed API. Due to the check and cast when using, I'm uncertain this is any better than having users manage extensions outside of the core.

Example Usage

import { Item, Extension, RegisterExtension } from './src/index';

class HiddenExtension {
    #item: Item
    #hidden: boolean

    constructor(item: Item, value: string) {
        this.#hidden = value === '1';
        this.#item = item;
    }

    isHidden():boolean {
        return this.#hidden;
    }

    hide() {
        this.#hidden = true;
        this.#item.setExtension('h', '1');
    }

    unhide() {
        this.#hidden = false;
        this.#item.setExtension('h', '0');
    }
}

RegisterExtension(
    ['h'],
    (item: Item, _: string, value: string):Extension => { 
        return new HiddenExtension(item, value);
    }
);

const item = new Item('This is hidden. h:1');

console.log(item.extensions());
// [ { key: 'h', value: '1' } ]

const annotated = item.toAnnotatedString();
console.log(annotated);
/*
{
  string: 'This is hidden. h:1',
  contexts: [],
  projects: [],
  extensions: [
    {
      string: 'h:1',
      object: HiddenExtension {},
      parsed: [Object],
      span: [Object]
    }
  ]
}
*/

annotated.extensions.forEach(ext => {
    if(ext.parsed.key === 'h' && ext.object !== null)  {
        (ext.object as HiddenExtension).unhide();
    }
})
console.log(item.extensions());
// [ { key: 'h', value: '0' } ]

console.log(item.toString());
// This is hidden. h:0

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[codecov[bot]]

Codecov Report

Base: 98.55% // Head: 96.74% // Decreases project coverage by -1.81% :warning:

Coverage data is based on head (ffafdd0) compared to base (1f7883e). Patch coverage: 64.51% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #36 +/- ## ========================================== - Coverage 98.55% 96.74% -1.82% ========================================== Files 1 2 +1 Lines 555 584 +29 Branches 47 48 +1 ========================================== + Hits 547 565 +18 - Misses 6 17 +11 Partials 2 2 ``` | [Impacted Files](https://codecov.io/gh/jmhobbs/jsTodoTxt/pull/36?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Hobbs) | Coverage Δ | | |---|---|---| | [src/Extensions.ts](https://codecov.io/gh/jmhobbs/jsTodoTxt/pull/36/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Hobbs#diff-c3JjL0V4dGVuc2lvbnMudHM=) | `54.16% <54.16%> (ø)` | | | [src/Item.ts](https://codecov.io/gh/jmhobbs/jsTodoTxt/pull/36/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Hobbs#diff-c3JjL0l0ZW0udHM=) | `98.57% <100.00%> (+0.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Hobbs). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Hobbs)

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[jmhobbs]

I think I prefer to move parsing and initialization outside of the core. My thought is to publish scoped packages like @jstodotxt/hidden-extension and opt in that way.