Narrow drive letter matching for splitdrive on Windows to one character, fixup docstring for joinpath (#58951)

Fixes #58929

Only a single letter followed by a colon is a valid drive prefix for
this format, any other length of string prior to a colon isn't a valid
drive in Windows:
https://learn.microsoft.com/en-us/dotnet/standard/io/file-path-formats

Also the docstring for `joinpath` rendered Windows paths as unescaped,
meaning they could not be copied and pasted on the REPL.

Author: BioTurboNick

base/path.jl

@@ -40,7 +40,7 @@ elseif Sys.iswindows()
         # Not a slash in either direction.
         N = raw"[^\\/]"
         # Drive letter, e.g. `C:`
-        drive = "$(N)+:"
+        drive = "$(N):"
         # UNC path, e.g. `\\server\share`
         unc = "$(S)$(S)$(N)+$(S)$(N)+"
         # Long drive letter, e.g. `\\?\C:`
@@ -357,7 +357,7 @@ the join of the preceding paths, then prior components are dropped.
 Note on Windows since there is a current directory for each drive, `joinpath("c:", "foo")`
 represents a path relative to the current directory on drive "c:" so this is equal to "c:foo",
 not "c:\\foo". Furthermore, `joinpath` treats this as a non-absolute path and ignores the drive
-letter casing, hence `joinpath("C:\\A","c:b") = "C:\\A\\b"`.
+letter casing, hence `joinpath("C:\\\\A","c:b") = "C:\\\\A\\\\b"`.
 
 # Examples
 ```jldoctest

test/path.jl

@@ -65,7 +65,7 @@
         end
 
         if Sys.iswindows()
-            @test joinpath(S("foo"),S("bar:baz")) == "bar:baz"
+            @test joinpath(S("foo"),S("D:bar")) == "D:bar"
             @test joinpath(S("C:"),S("foo"),S("D:"),S("bar")) == "D:bar"
             @test joinpath(S("C:"),S("foo"),S("D:bar"),S("baz")) == "D:bar$(sep)baz"
 
@@ -181,6 +181,9 @@
                 ("\\\\servername.com\\hello.world","\\filename.ext")
             @test splitdrive(S("C:\\foo\\bar")) ==
                 ("C:","\\foo\\bar")
+            # only single characters followed by a colon are drives
+            @test splitdrive(S("foo:bar")) ==
+                ("", "foo:bar")
         end
 
         @test splitext(S("")) == ("", "")