check files with names like [foo].html work on windows

[Rich-Harris]

For Sapper (which I'll move over to this organisation soon, once it's a less of a big pile of TODOs), I've settled on the following convention for declaring routes: a file like routes/post/[id].html translates to /post/:id — i.e. visiting /post/xyz causes [id].html to be rendered with params: { id: 'xyz' }.

Just need to make sure that Windows doesn't ruin the party, hence this PR.

[codecov-io]

Codecov Report

Merging #1010 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1010   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         115      115           
  Lines        4297     4297           
  Branches     1369     1369           
=======================================
  Hits         3956     3956           
  Misses        148      148           
  Partials      193      193

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[Rich-Harris]

it works! excellent.