realpath rejects `#`

Nobody has claimed this yet.

This section of hypertext.m2 is problematic:
```
fileExists' = pth -> (
if match("#",pth) then pth = substring(0,lastMatch#0#0,pth);
fileExists pth
)

-- TODO: phase this one out eventually
toURL = method()
toURL String := pth -> (
urlEncode if isAbsolutePath pth then concatenate(rootURI,
if fileExists' pth then realpath pth
```
Note that `fileExists` has been modified to allow extra `#` information after the file name, which is typically used by `FilePosition` and error messages.
There's a first basic issue which is, what if a filename contains a `#`?
In any case, a more fundamental problem is that `pth` is fed directly to `realpath` without removing the `#` part.
On my laptop, `realpath` rejects filenames with a `#` -- more precisely, it sends EONENT (i.e., file not found, as it should), though that signal is not correctly forwarded to M2, because stat(NULL) replaces ENOENT with EFAULT (“Bad address”), resulting in the cryptic error message
```
i18 : toURL "/home/pzinn/M2/M2/Macaulay2/m2/hypertext.m2#L451:C26-L453:C15"
stdio:18:5:(3):[1]: error: realpath failed: Bad address
```
I've only started encountering this problem recently (in fact, on the Macaulay2Web server, realpath seems happy with `#`), so not sure what exactly changed. but there's definitely an issue.

Inspect hypertext.m2 around fileExists' and toURL, then reproduce the reported toURL call with a path containing #. Determine how the # metadata is handled before realpath, including the filename case, and verify that the resulting error is no longer the reported failure.

Written by the indexing model from the issue text.