tools/webpack: Run webpack in the foreground.
tools/webpack: Run webpack in the foreground.

`tools/run-dev.py` already backgrounds `tools/webpack` (and deals with
cleaning it up on exit), so there’s no need for `tools/webpack` to
also background the actual `webpack` process.  But when running
`tools/webpack` by itself, it’s annoying to clean up the backgrounded
process manually.

Run `webpack` in the foreground, using `os.execvp` so we don’t waste
memory on an intermediate wrapper process.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>
andersk/zulip,andersk/zulip,tommyip/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,brainwane/zulip,kou/zulip,rht/zulip,showell/zulip,andersk/zulip,zulip/zulip,rht/zulip,rishig/zulip,timabbott/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,hackerkid/zulip,rht/zulip,tommyip/zulip,kou/zulip,shubhamdhama/zulip,showell/zulip,punchagan/zulip,shubhamdhama/zulip,zulip/zulip,eeshangarg/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,brainwane/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,synicalsyntax/zulip,rishig/zulip,showell/zulip,rishig/zulip,eeshangarg/zulip,rishig/zulip,rishig/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,hackerkid/zulip,kou/zulip,synicalsyntax/zulip,tommyip/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,kou/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,timabbott/zulip,shubhamdhama/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,andersk/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,showell/zulip,showell/zulip,zulip/zulip,tommyip/zulip,rishig/zulip,kou/zulip,andersk/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,brainwane/zulip,hackerkid/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,timabbott/zulip,punchagan/zulip,zulip/zulip,rishig/zulip,tommyip/zulip