Add ability to change .html extension for FallbackServer

[jimmymcpeter]

Is your feature request related to a problem? Please describe.
I am working with a site that uses .htm instead of .html for its file extension. The FallbackServer class only looks for .html files according to the following line --

lighthouse-ci/packages/cli/src/collect/fallback-server.js

Line 105 in 19c7ca6

const htmlFiles = files.filter(file => file.endsWith('.html')).map(file => ({file, depth: 0}));

Describe the solution you'd like
It'd be nice to be able to change the .html extension to .htm using a config parameter. Maybe even send an array of extensions.

Describe alternatives you've considered
Renaming all files to .html unfortunately breaks existing hyperlinks. Currently I have a script that overrides the FallbackServer class code from const htmlFiles = files.filter(file => file.endsWith('.html')).map(file => ({file, depth: 0})); to const htmlFiles = files.filter(file => file.endsWith('.htm')).map(file => ({file, depth: 0}));

Additional context
Documentation sites built using MadCap Flare almost always use the .htm extension.