Custom metadata and defines (documentation, completion)

[kLabz]

See #10844

Note: can also make use of HaxeFoundation/haxelib#573 so that libs can declare files similar to our own meta.json / define.json files directly in their haxelib.json.

• Get Add support for customMetas and customDefines fields haxelib#573 merged

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Allows user (and mostly libs) to declare their custom medatada and defines in a similar way as we do with meta.json and define.json, so that editors can provide completion / documentation for those.

Adds 4 haxe.macro.Compiler endpoints:

• registerMetadataDescriptionFile to register a file similar to meta.json
• registerDefinesDescriptionFile to register a file similar to define.json
• registerCustomMetadata to add documentation about a single custom metadata
• registerCustomDefine to add documentation about a single custom define

Adds two haxe compiler arguments:

• --help-user-metas to display user-defined metadata documentation for current compilation context
• --help-user-defines to display user-defined defines documentation for current compilation context

Example usage (with partial documentation; can also use a build.hxml file and grab documentation from libs):

$ haxe -lib react-next --help-user-metas
 @:noPublicProps: Disallow public props for this component when used in jsx.
                  (from react-next)
 @:publicProps  : (<TProps>) Set public props type to ensure jsx type checking.
                  (from react-next)
 @:wrap         : (<HOC>) Wrap current class component in a HOC. (from react-next)

$ haxe -lib react-next --help-user-defines
 react-global          : Use window.React instead of require() from
                         modular/webpack. (from react-next)
 react-hot             : Adds some data needed for react hot reloading with
                         modular/webpack. (from react-next)
 react-no-inline       : Skip React.createElement inlining optimization.
                         (from react-next)
 react-runtime-warnings: Enable some runtime warnings for debug purpose.
                         (from react-next)

There are likely things that could be written in a better way, have a better name, etc.
Please give some feedback :)

[Gama11]

Haven't checked the code, so maybe you already did this, but: There should probably be some flag in the display API to tell apart custom defines / metas from built-in ones, so the editor can choose to render them a bit differently.

Actually, even better, maybe some kind of origin field, which could be a library name for instance, so the editor can show from which library a meta originates.

[Simn]

Thank you, and good job! This is a really neat addition.