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

[ben12]

PR Checklist

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• 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)
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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!