Add option for symbol matching logic. (#1515)

The no_symbol_match makes command line completion a lot less useful. It
disables any matches for file names with symbols in them. This prevents
completing things like ":b foo/bar" to ":b foo/bar.txt" or ":b foo_" to
":b baz/foo_bar.txt". Add an option `disallow_symbol_nonprefix_matching`
that prevents a match if it contains a symbol and isn't a prefix match.
Make that option the default. Add the option to documentation and tests.
Add to the examples for command line setup disabling the option.

lua/cmp/types/cmp.lua

@@ -137,6 +137,7 @@ cmp.ItemField = {
 ---@field public disallow_partial_fuzzy_matching boolean
 ---@field public disallow_partial_matching boolean
 ---@field public disallow_prefix_unmatching boolean
+---@field public disallow_symbol_nonprefix_matching boolean
 
 ---@class cmp.SortingConfig
 ---@field public priority_weight integer