Type based autocompletion

Rough outline of what work to do in what order:

• Type based completion for function arguments https://github.com/rescript-lang/rescript-vscode/pull/668
• Complete booleans, options, variant constructors https://github.com/rescript-lang/rescript-vscode/pull/668
• Type based completion for JSX props https://github.com/rescript-lang/rescript-vscode/pull/667
• Support snippets in completion items https://github.com/rescript-lang/rescript-vscode/pull/668
• Complete polyvariants https://github.com/rescript-lang/rescript-vscode/pull/669
• Complete record construction https://github.com/rescript-lang/rescript-vscode/pull/682
• Complete tuples https://github.com/rescript-lang/rescript-vscode/pull/668
• Complete nested expressions when constructing values https://github.com/rescript-lang/rescript-vscode/pull/682
• Complete patterns in switches https://github.com/rescript-lang/rescript-vscode/pull/670
• Complete patterns when destructing https://github.com/rescript-lang/rescript-vscode/pull/670
• Expand completion option items when appropriate. Some(_) -> Some(true) and Some(false) for example https://github.com/rescript-lang/rescript-vscode/pull/690
• Piggy back on Ppat_any (_) to trigger completion https://github.com/rescript-lang/rescript-vscode/pull/692
• Support inline records in completion https://github.com/rescript-lang/rescript-vscode/pull/695
• Handle optional record fields https://github.com/rescript-lang/rescript-vscode/pull/691
• Complete functions (insert empty function) https://github.com/rescript-lang/rescript-vscode/pull/701
• Complete lowercase jsx props too https://github.com/rescript-lang/rescript-vscode/pull/702
• Complete using type annotations https://github.com/rescript-lang/rescript-vscode/pull/711
• "Seen idents" for more patterns
• Leverage "filterText" to ensure module names aren't included when filtering completion list (unless only searching for modules)
• Do not add "any" when inserting variant payloads in expressions

Things to figure out

• Qualify JSX V3 constructors Not doing this, let people upgrade to jsx v4 to fix it
• Fix insertText not being used unless there's snippet support
• Figure out more instances where we want to do typed expr or pattern completion

Future

• Figure out when to expand options in JSX props (read more in description of https://github.com/rescript-lang/rescript-vscode/pull/667). Will need to parse the React component make function and look whether prop is optional or not.

Start with the three unchecked checklist items and the “Things to figure out” section, then read the linked pull requests for context on the completed completion work. Done means implementing the selected remaining completion behaviors and resolving the listed open questions, with coverage added wherever the existing project conventions require it.