Upgrade and fix function signature computation, and optimize performance

[vquelque]

This PR follows the PR in solidity-workspace here. Note that before merging this PR, the new solidity-workspace package must be published, and the dependency in this package bumped, as some breaking changes were introduced in solidity-workspace. This PR focuses on fixing the function signature feature and improving the overall performance of the extension. Note that more work can be done to further improve the performances.

This PR has the following objectives:

• It upgrades the feature that generates function signatures, so that the signatures are generated from the AST
• It moves several features - such as the resolution of inherited identifiers - to solidity-workspace
• It adds a debounce function to rate-limit the requests made to the parser
• It refactors the CodeLenses, and the symbol provider, to optimize their performances
• It has various smaller improvements intended to optimize the overall performance of the extension.

To enforce a consistent code style, the sources were formatted with Prettier.

[tintinweb]

included in #128