Convert from 1- to 0-based indexing for TypeScript completion spans

[aozgaa]

Fixes an issue where module and triple slash completions had an off-by-one error with the indexing of text spans. Corresponding issues in the TypeScript repo are:

microsoft/TypeScript#12165
microsoft/TypeScript#11462

[mention-bot]

@aozgaa, thanks for your PR! By analyzing the history of the files in this pull request, we identified @dbaeumer, @egamma and @jrieken to be potential reviewers.

[msftclas]

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[billti]

Looks good. I can see in the protocol docs their API expects 0-based (see https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#position ). You can see in other operations such as formatting ( https://github.com/Microsoft/vscode/blob/master/extensions/typescript/src/features/formattingProvider.ts#L172 ) where they are converting from the TypeScript 1-based positions - so this seems consistent with existing features.

[dbaeumer]

@aozgaa thanks for providing this!