CI: Add Lychee CI test to check for broken links

closes: #17879

Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>

ci/lychee.toml

@@ -0,0 +1,47 @@
+#############################  Runtime  #############################
+
+# Reasonable concurrency to avoid rate-limiting small hosts.
+max_concurrency = 14
+
+# File extensions to scan
+extensions = ["c", "h", "md", "html", "txt"]
+
+#############################  Requests  ############################
+
+# Accepted status codes
+# - Include 429 (Too Many Requests) to avoid marking throttled links as dead.
+accept = ["100..=103", "200..=299", "429"]
+
+# Retry each request a few times on transient network errors
+retries = 2
+retry_wait_time = 2
+
+# Timeout per request in seconds
+timeout = 20
+
+#############################  Exclusions  ##########################
+
+exclude = [
+  '^https?://sys/file$',
+  '^file://.*',
+  '^https?://(www\.)?badges\.debian\.net/.*$',
+  '^https?://(www\.)?repology\.org/.*$',
+  '^https?://scan\.coverity\.com/.*$',
+  '^https?://(www\.)?img\.shields\.io/.*$',
+  '^https?://(www\.)?fossies\.org/.*$',
+  '^https?://(www\.)?adobe\.com.*$',
+  '^https?://(www\.)?google\.com.*$',
+  '^https?://(www\.)?forum\.vmssoftware\.com/.*$',
+  '^http://vim\.sourceforge\.net/scripts/script\.php\?script_id=40$',
+  '^https://is\.gd/.*$',
+  '^https://wiki\.contextgarden\.net/Vim$',
+  '^https://github\.com/you/foobar\.git$',
+  '^https?://flatassembler\.net',
+  '^https?://www\.fleiner\.com/vim/download.html',
+]
+
+# Lychee only reads UTF-8 files
+exclude_path = [
+  'src/po',
+  'src/testdir',
+]