Type based autocompletion

[zth]

Rough outline of what work to do in what order:

• [x] Type based completion for function arguments https://github.com/rescript-lang/rescript-vscode/pull/668
• [x] Complete booleans, options, variant constructors https://github.com/rescript-lang/rescript-vscode/pull/668
• [x] Type based completion for JSX props https://github.com/rescript-lang/rescript-vscode/pull/667
• [x] Support snippets in completion items https://github.com/rescript-lang/rescript-vscode/pull/668
• [x] Complete polyvariants https://github.com/rescript-lang/rescript-vscode/pull/669
• [x] Complete record construction https://github.com/rescript-lang/rescript-vscode/pull/682
• [x] Complete tuples https://github.com/rescript-lang/rescript-vscode/pull/668
• [x] Complete nested expressions when constructing values https://github.com/rescript-lang/rescript-vscode/pull/682
• [x] Complete patterns in switches https://github.com/rescript-lang/rescript-vscode/pull/670
• [x] Complete patterns when destructing https://github.com/rescript-lang/rescript-vscode/pull/670
• [x] Expand completion option items when appropriate. Some(_) -> Some(true) and Some(false) for example https://github.com/rescript-lang/rescript-vscode/pull/690
• [x] Piggy back on Ppat_any (_) to trigger completion https://github.com/rescript-lang/rescript-vscode/pull/692
• [x] Support inline records in completion https://github.com/rescript-lang/rescript-vscode/pull/695
• [x] Handle optional record fields https://github.com/rescript-lang/rescript-vscode/pull/691
• [x] Complete functions (insert empty function) https://github.com/rescript-lang/rescript-vscode/pull/701
• [x] Complete lowercase jsx props too https://github.com/rescript-lang/rescript-vscode/pull/702
• [x] 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.

[zth]

Non-exhaustive (and living) list of things we'd like to explore:

• Complete variant/polyvariant members when assigning a labelled argument
• Complete variant/polyvariant members when passing an unnamed function parameter eg someFunc(<complete>) (is this possible given currying etc?)
• Complete variant/polyvariant members when assigning a JSX prop
• Complete missing record fields as a record is constructed
• Complete missing record fields as a record is destructured
• Complete variant/polyvariant members in switch branches

If you have ideas not listed here, please feel free to detail them!