Add support for JSONC to dotfiles merge entries

[TimonLukas]

Hi! I'm trying to move my VSCode settings.json into my managed dotfiles, using the merge feature. Unfortunately the VSCode devs decided not to follow the JSON standard, so comments and dangling commas in settings.json trip Mise up[1].

Would it be possible to add support for merging JSONC files? Since the settings.json case can't be recognized based on the file extension, a manual format override would also be necessary (unless JSON files should fall back to the JSONC parser, but that feels error-prone).

Blocks can probably also be made to work because comments are allowed, but the hierarchical nature of JSON still makes this a bit error-prone.

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[1]: By the way, I noticed that a source with invalid JSON causes an error that doesn't actually mention where the issue happens:

mise ERROR edits: cannot apply these entries, fix them manually:
  [dotfiles]."~/.config/Code/User/settings.json/shared": invalid merge source
mise ERROR Version: 2026.10.21 linux-x64 (2026-10-27)

If the source has valid JSON, but the target doesn't, the error message is much more specific:

mise ERROR edits: cannot apply these entries, fix them manually:
  "~/.config/Code/User/settings.json" (merge:shared): failed to parse the JSON target: key must be a string at line 6 column 3
mise ERROR Version: 2026.10.21 linux-x64 (2026-10-27)

[jdx]

Thanks for the detailed write-up, and for the note about the error message.

I opened #15077, which adds JSONC support to merge entries. For settings.json set format = "jsonc":

[dotfiles]
"~/.config/Code/User/settings.json/shared" = { source = "vscode/shared.json", merge = true, format = "jsonc" }

The target is edited in place, so comments, trailing commas and keys you don't own stay as written. The source can contain comments too. A .jsonc target is detected by extension, and a plain .json target without format is still strict JSON, so existing setups don't change.

The error you hit also changes: an invalid source now names the file and the parse error, for example invalid merge source /path/shared.json: failed to parse the JSON source: ..., instead of just invalid merge source.

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