feat(developer): Add support for building a folder to kmc

Relates to #2761.

kmc now supports passing in a folder and building all Keyman source
files found in the folder, even if a .kpj is not present in the folder.

If a .kpj file exists, `KeymanDeveloperProject/Options/Version` will be
checked to determine behaviour. If this field is missing or "1.0", then
the classical behaviour persists.

If the Version field is "2.0", the `Files` key is ignored, and .kmn,
.xml, .model.ts, and .kps files under `sourcePath` are enumerated and
added to the in-memory project for build.

If a .kpj is not present in the folder, a virtual project is constructed
in memory with the following defaults:

* `version`: `2.0`
* `buildPath`: `$PROJECTPATH/build`
* `sourcePath`: `$PROJECTPATH/source`
* `compilerWarningsAsErrors`: `false`
* `warnDeprecatedCode`: `true`
* `checkFilenameConventions`: `true`

It is still considered an error to include both a keyboard and a model
in the same file, but a version 2.0 project will not incl...