add .gitattributes file for end-of-line normalisation

This should prevent accidentally committing files with CRLF line endings to the repo.

related: https://github.com/spesmilo/electrum/issues/8025

see https://git-scm.com/docs/gitattributes#_end_of_line_conversion :
> If you want to ensure that text files that any contributor introduces to the repository have their line endings normalized, you can set the text attribute to "auto" for all files.
> ```
> * text=auto
> ```
>
> From a clean working directory:
> $ echo "* text=auto" >.gitattributes
> $ git add --renormalize .
> $ git status    # Show files that will be normalized
> $ git commit -m "Introduce end-of-line normalization"

-----

I have tried to find some large cross-platform projects that use git for EOL normalisation.
Notably the golang/go project recommends against it, but the others have it in some form:

4ec9ed8fde/.gitattributes
160bb0e66b/.gitattributes
7fda097623/.gitattributes
103a5c98b8/.gitattributes
0990046d8d/.gitattributes
1c2dc79f51/.gitattributes
0d65a985b9/.gitattributes
d3bafb8b6e/.gitattributes

.gitattributes

@@ -0,0 +1,12 @@
+# Auto detect text files and perform end-of-line normalization (to LF)
+*        text=auto
+
+# These Windows files should have CRLF line endings in checkout
+*.bat    text eol=crlf
+*.ps1    text eol=crlf
+
+# Never perform LF normalization on these files
+*.ico    binary
+*.jar    binary
+*.png    binary
+*.zip    binary