GHA/checksrc: expand spellcheck, fix issues found

- codespell: break logic out into its own runnable script. Allowing
  to run it on local machines.
- codespell: install via `pip`, bump to latest version.
- codespell: show version number in CI log.
- codespell: drop no longer needed word exception: `msdos`.
- codespell: include all curl source tree, except `packages` and
  `winbuild`. Drop an obsolete file exclusion.
- add new spellchecker job using the `typos` tool. It includes
  the codespell dictionary and a couple more. Use linuxbrew to install
  it. This takes 10 seconds, while installing via `cargo` from source
  would take over a minute.
- codespell: introduce an inline ignore filter compatible with `cspell`
  Make `typos` recognize it, too. Move single exceptions inline.

Fix new typos found. Also rename variables and words to keep
spellchecking exceptions at minumum. This involves touching some tests.
Also switch base64 strings to `%b64[]` to avoid false positives.

Ref: https://github.com/crate-ci/typos/blob/master/docs/reference.md
Ref: https://github.com/codespell-project/codespell?tab=readme-ov-file#inline-ignore
Ref: https://github.com/codespell-project/codespell/issues/1212#issuecomment-1721152455
Ref: https://cspell.org/docs/Configuration/document-settings

Closes #17905

.github/scripts/codespell.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+set -eu
+
+cd "$(dirname "${0}")"/../..
+
+# shellcheck disable=SC2046
+codespell \
+  --skip '.github/scripts/spellcheck.words' \
+  --skip '.github/scripts/typos.toml' \
+  --skip 'docs/THANKS' \
+  --skip 'packages/*' \
+  --skip 'scripts/wcurl' \
+  --skip 'winbuild/*' \
+  --ignore-regex '.*spellchecker:disable-line' \
+  --ignore-words '.github/scripts/codespell-ignore.txt' \
+  $(git ls-files)