fix(#621) Plural translation starting and ending with a variables fails

[ben12]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/transloco/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #621

What is the new behavior?

To allow variable at start or end of plural translations (ex: {count, plural, =1 {1 person} other {{count} people}}), we add some forbidden characters for parameters name:

export interface TranslocoConfig {
[...]
  interpolation?: [start: string, end: string, forbiddenChars?: string];
}

Default configuration become:

export const defaultConfig: Omit<
  Required<TranslocoConfig>,
  'scopeMapping' | 'fallbackLang'
> = {
[...]
  interpolation: ['{{', '}}', '{}'],
};

So in {count, plural, =1 {1 person} other {{count} people}}, {{count} people}} will no longer be processed as variable.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Translation keys using braces are now forbidden.

Other information

I have implemented the check for forbidden key characters in transloco-validator with its unit tests.

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[ben12]

@shaharkazaz Ready for review

[shaharkazaz]

@ben12 Thank you for the PR and the heads up, I missed this PR.
IDK when I'll be able to review but I'll try to reach it as soon as I have the time!