Design versioning strategy for MSI pre-release & fix the MSI generator script

MSI versioning is limited to x.x.x (major.minor.build) and as such does not support pre-release versions, or semver standards.

Today, the MSI script is broken as it assumes MSI versioning allows 4 numbers (x.x.x.x):

https://github.com/Azure/azure-functions-core-tools/blob/78afd8b84c8e31f0ddac570ba9e8128eefbd3d4a/eng/scripts/generate-msi-files.ps1#L66-L67

(It also doensn't support the `-preview` suffix but that's besides the point)

This hasn't been an issue so far as we have never released a preview MSI package before, however if we ever intend to in future, this issue needs to be addressed before hand.

#### Tasks

- [ ] Design a versioning approach that supports pre-release versioning

> An potential solution could be using the ado build number for all MSI versions (as suggested by the MSI version docs e.g. 4.3.235164), never including patch. Or designing a schema such as [this](https://github.com/semver/semver/issues/332#issuecomment-283644851) one.

- [ ] Update the `generate-msi-files.ps1` to adhear to the new versioning standard

If we decide that we do _not_ want to ever support MSI pre-releases (that's also fine as we have many other release channels), we should at the very least fix the current msi generator script to ensure a valid MSI version when any suffix is included.

Start with eng/scripts/generate-msi-files.ps1 at the linked lines 66-67 and review the MSI versioning limitation described in the issue. Decide and document a versioning standard that handles pre-release versions, then update the generator so its output follows that standard; if pre-releases remain unsupported, ensure suffix-containing versions still produce valid MSI versions.

Written by the indexing model from the issue text.