Support version-specific Manifest

[quachpas]

Fixes #1316. This is in parallel with julia-vscode/SymbolServer.jl#303. Same comments apply.

This adds to the list of manifest candidates the version-specific Manifest based on the Julia version of the LSP.
There is a strong assumption because in load_project_packages_into_store!, SymbolServer.jl will now prefer in order: Manifest-vX.XX.toml, Manifest.toml, JuliaManifest.toml.
I am not sure whether this is acceptable. If this is approved, then I will documentation at the VS Code extension docs and mention this in the changelog.

For every PR, please check the following:

• End-user documentation check. If this PR requires end-user documentation in the Julia VS Code extension docs, please add that at https://github.com/julia-vscode/docs.
• Changelog mention. If this PR should be mentioned in the CHANGELOG for the Julia VS Code extension, please open a PR against https://github.com/julia-vscode/julia-vscode/blob/master/CHANGELOG.md with those changes.