Add prepareRename command

[zth]

This adds a prepareRename command to the analysis bin that the editor tooling can use to get quick feedback for whether the target thing can be renamed or not.

Background for this is that our current rename functionality feels slow, and that's because it does all the heavy lifting of figuring out all places a rename needs to be applied, plus producing the actual text edits, twice - once to simply check if something can be renamed, and then again to do the actual rename. The command added in this PR replaces the first with a light weight super fast command that only looks at the untyped AST.

Will need adjustments in the editor tooling repo too for this to actually work.

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commit: cdc8ac1

[mediremi]

LGTM ✔️