feat(completions): autocomplete for function types

[juleswritescode]

Autocompletions for functions! Some rules that are applied:

• if we're in a select clause, we'll prefer function items over table items
• if we're in an "invocation" (parentheses after the current token), we'll prefer functions
• if we're in a from clause, we'll slightly prefer table items

I refactored the CompletionRelevance again; now we can update the score based on the completion item score.

I also added docs to the SchemaCache's Function and changed the query. Some properties were Options, even though they should always be present: id, schema etc. – please verify if I made any wrong assumptions 🙌

Some extra stuff:

• added a few additional test helpers (only compiled when we're running tests)
• changed the CompletionItem.preselected from Option<bool> to a simple bool
• added a ClauseType enum

Note: I had to comment out an alphanumeric table test. The query generates a tree-sitter tree with an ERROR, and the current node has an end_byte() that is at position 17, while the query only has a length of 14... will check in a separate PR, not sure why it even worked before 🤷

[psteinroe]

lgtm! Two nits, but we can merge 🙏🏼

[psteinroe]

@juleswritescode, sorry, can you fix the conflict again? Should have merge the other pr after this one

[juleswritescode]

@juleswritescode, sorry, can you fix the conflict again? Should have merge the other pr after this one

Check!!