fix(snippets): ensure proper espcaping of special chars (#2239)

Previously, when working with the built-in snippet engine,
placeholders like `TM_*` or `CLIPBOARD` were populated blindly without
escaping special characters. This caused parsing failures, especially
for LaTeX snippets with extensive use of these chars.

This change ensures proper escaping according to LSP snippet grammar
EBNF conventions.

Closes #2072
Related #2028

Author: soifou

lua/blink/cmp/sources/snippets/default/registry.lua

@@ -178,6 +178,11 @@ function registry:expand_vars(snippet, cache_key)
       end
 
       if replacement then
+        -- Escape special chars according to the snippet grammar EBNF conventions
+        replacement = replacement:gsub('\\', '\\\\') -- Escape backslashes first!
+        replacement = replacement:gsub('%$', '\\$')
+        replacement = replacement:gsub('}', '\\}')
+
         -- Escape % characters (otherwise fails with strings like `%20`)
         local escaped = replacement:gsub('%%', '%%%%')