Add template for reporting 'better-error' issues (#18672)

Add a new issue template for reporting unhelpful/confusing error
messages.

[skip-ci]

.github/ISSUE_TEMPLATE/improve-error.md

@@ -0,0 +1,55 @@
+---
+name: "\U0001F615 Error/Warning message report"
+about: Report an error/warning message that was confusing/unhelpful
+title: ''
+labels: itype:enhancement, area:reporting, better-errors, stat:needs triage
+assignees: ''
+
+---
+
+## Compiler version
+
+If you're not sure what version you're using, run `print scalaVersion` from sbt
+(if you're running scalac manually, use `scalac -version` instead).
+
+## Minimized example
+
+<!--
+This code should be self-contained, reproducible (i.e. produces the expected error/warning message) and as small as possible.
+
+Ideally, we should be able to just copy this code in a file and run `scalac` (and maybe `scala`) to reproduce the issue.
+
+For a good example, see https://github.com/lampepfl/dotty/issues/18657
+-->
+
+```Scala
+printl("hello, world")
+```
+
+## Output Error/Warning message
+
+<!--
+Here, please provide the produced error/warning message that is confusing/unhelpful, etc.
+
+for example:
+-->
+
+```scala
+-- [E006] Not Found Error: ----------------
+1 |printl("hello, world")
+  |^^^^^^
+  |Not found: printl
+1 error found
+```
+
+## Why this Error/Warning was not helpful
+
+<!-- For a good example, see https://github.com/lampepfl/dotty/issues/18657 -->
+
+The message was unhelpful because...
+
+## Suggested improvement
+
+<!-- For a good example, see https://github.com/lampepfl/dotty/issues/18657 -->
+
+It could be made more helpful by...