Navigation is blocked by parser failures

[kanej]

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If a user edits a file and leaves the code such that the solidity-parser errors, then we lose code navigation in that file.

Reproduction Steps

https://user-images.githubusercontent.com/24030/173074968-43eaebd0-7657-49e7-aa05-9b3aa543680a.mov

Options

1. use a more tolerant parser (slang)
2. make solidity-parser more tolerant
3. retain the previously working ast, and use that for navigation

[github-actions[bot]]

This issue is also being tracked on Linear.

We use Linear to manage our development process, but we keep the conversations on Github.

LINEAR-ID: a3aad1a0-a0cb-413e-92d0-4757c5091526