Add support for build.mill and build.mill.scala

[lolgab]

Companion PR of scalameta/metals#6752

[tgodzik]

Looks good! Let's wait for the metals PR first.

[ckipp01]

Just a note for clients that also have logic to detect the root based off build files, they should also account for this.

[lolgab]

Just a note for clients that also have logic to detect the root based off build files, they should also account for this.

I'm not sure I understand. Can you elaborate?

[ckipp01]

Just a note for clients that also have logic to detect the root based off build files, they should also account for this.

I'm not sure I understand. Can you elaborate?

Sure, and this is more of a note just for client maintainers, not necessarily for you. But in some clients, like Neovim and other terminal based editors, the idea of a "workspace" is sort of artificial. This causes the client to have to have some sort of logic to determine where the workspace root is. For example in nvim-metals we have some logic where we take a pre-set set of patterns that we look for and then we have some logic about traversing down to account for nested builds in different tools, etc. So even though this changes here in Metals, it's good, but this doesn't 100% mean it will actually work in some clients unless they also account for these filetypes.

EDIT: I'm now seeing that this pr is in the VS Code repo, not the actual metals repo... totally my mistake, so just ignore me.