Rollup merge of #79698 - m-ou-se:libs-tracking-issue-template, r=KodrAus

Add tracking issue template for library features.

This adds a issue template for a library tracking issue.

There's already a template for tracking issues, but it's mostly geared towards compiler/language features. A separate template makes it a bit easier to make sure it matches with the process we use for library changes.

Main differences:
- Added a note about how small library features can be added without RFC, and removed the parts that assume there's an RFC.
- Merged the 'Steps' and 'History' sections: Library features are often small enough that there's no multiple steps planned ahead of time.
- Removed the section about avoiding large discussions and opening separate issues for problems with the feature. Library features are usually focussed enough that the discussion about a feature is best kept together in the tracking issue.
- Removed links to the rustc-dev-guide, which are specific to changes in the compiler and language.

.github/ISSUE_TEMPLATE/library_tracking_issue.md

@@ -0,0 +1,63 @@
+---
+name: Library Tracking Issue
+about: A tracking issue for an unstable library feature.
+title: Tracking Issue for XXX
+labels: C-tracking-issue T-libs
+---
+<!--
+Thank you for creating a tracking issue!
+
+Tracking issues are for tracking a feature from implementation to stabilization.
+
+Make sure to include the relevant RFC for the feature if it has one.
+
+If the new feature is small, it may be fine to skip the RFC process. In that
+case, you can use use `issue = "none"` in your initial implementation PR. The
+reviewer will ask you to open a tracking issue if they agree your feature can be
+added without an RFC.
+-->
+
+Feature gate: `#![feature(...)]`
+
+This is a tracking issue for ...
+
+<!--
+Include a short description of the feature.
+-->
+
+### Public API
+
+<!--
+For most library features, it'd be useful to include a summarized version of the public API.
+(E.g. just the public function signatures without their doc comments or implementation.)
+-->
+
+```rust
+...
+```
+
+### Steps / History
+
+<!--
+In the simplest case, this is a PR implementing the feature followed by a PR
+that stabilises the feature. However it's not uncommon for the feature to be
+changed before stabilization. For larger features, the implementation could be
+split up in multiple steps.
+-->
+
+- [ ] Implementation: ...
+- [ ] Stabilization PR
+
+### Unresolved Questions
+
+<!--
+Include any open questions that need to be answered before the feature can be
+stabilised. If multiple (unrelated) big questions come up, it can be a good idea
+to open a separate issue for each, to make it easier to keep track of the
+discussions.
+
+It's useful to link any relevant discussions and conclusions (whether on GitHub,
+Zulip, or the internals forum) here.
+-->
+
+- None yet.