Parser/stringifier tests

When parsing and stringifying, the module should check for valid path syntax.

Including:
- [ ] Begins with an M command
- [ ] Each command has the correct number of parameters
- [ ] Arc command large-arc-flag and sweep-flag should be 1 or 0
- [ ] String does not contain other characters

Locate the module's parser and stringifier entry points and read how SVG path commands are currently handled. Add coverage for the listed validity rules: an initial M command, correct parameter counts, binary arc flags, and rejection of other characters; done means the parser and stringifier tests cover each case.

Written by the indexing model from the issue text.