Expression AST builder

[streamich]

Create utilities for building expression chain AST. visualizations plugin will use those utilities in build_pipeline.ts to construct expression chain AST for each visualization, instead of concatenating strings to construct raw expression pipline string.

• AST builder utilities to construct all parts of expression AST:
  • Chain;
  • Expression function;
  • Arguments;
    • Sub-expressions in arguments;
• Export builder statically for visualization plugin to consume.

Parent issue: #46909

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[lukeelmers]

We should get input from the Canvas team on the design of this to make sure it is something they could use too -- last time I had discussed with Joe, he had mentioned they considered writing something like this for manipulating the AST via the Canvas editor, but never ended up doing it and stuck with manually editing the AST. Things may have changed since then, but worth looking into.

[lukeelmers]

For reference this was an exploration I did way back when about how to make it easier to manipulate the expression strings.

The thing we decided from this POC was that we definitely don't want to keep manipulating the string directly like this, but rather build something to work with the AST directly as you describe here. This has a bunch of obvious benefits including type safety.

I never got around to updating that POC to work with the AST, but thought I'd share here in case it is useful in any way.