"pxt serve" conflates hostname with network address

[xobs]

Bug Template

Describe the bug
The documentation for pxt serve lists a --hostname parameter that can be used to specify the hostname to listen for. This can be useful when running pxt serve in a Docker container, or in any form where a remote person may want to connect but the external hostname is different from the local hostname.

To Reproduce
Steps to reproduce the behavior:

1. Map an external address to your host (for example, through Docker, or from behind a NAT)
2. Give the external address a hostname
3. Run pxt serve --hostname [external-hostname]
4. See error

Expected behavior
The hostname is different from the bind address. The server should either have a separate flag for "bind address", or it should bind to "0.0.0.0".

Screenshots

[8:12:15 PM] D:/Code/makecode/pxt-chibitronics> pxt serve --hostname makecode-dev.xobs.io
Using target PXT/chibitronics with build engine platformio
  Target dir:   D:\Code\makecode\pxt-chibitronics
  PXT Core dir: D:\Code\makecode\pxt-chibitronics\node_modules\pxt-core
log strings: 34 files; 0 strings -> sim-strings.json
error TS6053: File 'D:/Code/makecode/pxt-chibitronics/node_modules/pxt-core/built/lib.dom.d.ts' not found.
error TS6053: File 'D:/Code/makecode/pxt-chibitronics/node_modules/pxt-core/built/lib.dom.iterable.d.ts' not found.
error TS6053: File 'D:/Code/makecode/pxt-chibitronics/node_modules/pxt-core/built/lib.es2015.d.ts' not found.
error TS6053: File 'D:/Code/makecode/pxt-chibitronics/node_modules/pxt-core/built/lib.scripthost.d.ts' not found.
error TS2318: Cannot find global type 'Array'.
error TS2318: Cannot find global type 'Boolean'.
error TS2318: Cannot find global type 'Function'.
error TS2318: Cannot find global type 'IArguments'.
error TS2318: Cannot find global type 'Number'.
error TS2318: Cannot find global type 'Object'.
error TS2318: Cannot find global type 'RegExp'.
error TS2318: Cannot find global type 'String'.
building target.json in D:\Code\makecode\pxt-chibitronics...
building docs in libs/core
Package built; written to binary.hex; size: 0
generated _locales\core-strings.json; size=4168
generated _locales\core-jsdoc-strings.json; size=13819
building docs in libs/scope
Package built; written to binary.hex; size: 0
generated _locales\scope-strings.json; size=160
generated _locales\scope-jsdoc-strings.json; size=370
building libs/core
  skip native build of non-project
building libs/scope
  skip native build of non-project
building libs\blocksprj
target.json built.
building sim...
[run] cd sim; node ../node_modules/typescript/bin/tsc
building cmds...
[run] cd cmds; node ../node_modules/typescript/bin/tsc
building editor...
[run] cd editor; node ../node_modules/typescript/bin/tsc
watching D:\Code\makecode\pxt-chibitronics\node_modules\pxt-core, libs/core, libs/scope, theme, theme\site\globals, editor, sim, sim\public, sim\state, sim\visuals, sim, cmds, editor...
Starting server in D:\Code\makecode\pxt-chibitronics
With pxt core at D:\Code\makecode\pxt-chibitronics\node_modules\pxt-core
starting local ws server at 3233...
INTERNAL ERROR: Error: listen EADDRNOTAVAIL: address not available 132.147.96.146:3233
    at Server.setupListenHandle [as _listen2] (net.js:1239:19)
    at listenInCluster (net.js:1304:12)
    at GetAddrInfoReqWrap.doListen [as callback] (net.js:1443:7)
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:63:10)
[8:12:41 PM] D:/Code/makecode/pxt-chibitronics>

Additional context
This is work that's being done in order to share C++ changes with designers who do not have a pxt development environment installed.