[ci] Forbid CRLF and trailing whitespace in tracked files.

[vgvassilev]

Add a small workflow that runs on pull_request and rejects two classes of text-hygiene problems before they land:

• Any committed file with CRLF or mixed line endings, detected via git ls-files --eol's index column. Binary files get i/-text and are skipped automatically. The check looks at the index column rather than the working tree so a contributor's local CRLF on a Windows checkout (without core.autocrlf configured) doesn't trip CI -- only what actually gets committed counts.
• Any trailing whitespace, conflict marker, or whitespace-style error on a line the PR adds or changes, detected via git diff --check $base...HEAD. This is the same logic git uses to render whitespace warnings in git diff output, so the message a contributor sees in CI matches what they would see locally. Diff-scoped on purpose: ~30 files in the tree have pre-existing trailing whitespace that we don't want to touch in this PR; the rule applies to new additions only, mirroring how clang-format.yml checks the diff rather than the whole tree.

Checkout uses fetch-depth: 0 and ref: head.sha so the diff base (pull_request.base.sha) is in local history and HEAD points at the PR tip rather than the synthetic merge ref.

Five files in the action set committed historically with CRLF (Install_Dependencies, Save_PR_Info, Select_Default_Build_Type, Setup_Compiler action.yml, plus .github/pull_request_template.md) are normalized to LF in the same commit -- without the conversion the new CRLF check would fail on its first run. The LF rewrite makes every line of each file appear "added" in the diff, which also surfaces two pre-existing trailing-whitespace lines (Install_Dependencies/action.yml:51,
Select_Default_Build_Type/action.yml:14); strip those too so the diff-scoped trailing-ws check passes as well.

A .gitattributes with * text=auto eol=lf would catch CRLF at commit time on contributor machines and is a reasonable follow-up; this CI check stays as the backstop regardless.