Allow using basedir as a root base directory for files and paths.

[tritao]

What does this PR do?

This adds some improvements to basedir so it can be used to specify a base directory for file, path and directory fields.

How does this PR change Premake's behavior?

Previously it only could be used for tokens, but now it can also affects paths, so depending on the order it's set, it might provide different results.

Anything else we should know?

Another option would be to go with a new srcdir, that also seems like a reasonable approach to take.

So feel free to provide some feedback if you have preferences how this should work.

Did you check all the boxes?

• [x] Focus on a single fix or feature; remove any unrelated formatting or code changes
• [x] Add unit tests showing fix or feature works; all tests pass
• [x] Mention any related issues (put closes #XXXX in comment to auto-close issue when PR is merged)
• [x] Follow our coding conventions
• [x] Minimize the number of commits
• [x] Align documentation to your changes

Closes https://github.com/premake/premake-core/issues/2129.