feat(renovate): track mise gitlab: backend tool pins

What

Adds a Renovate customManager for mise.toml tool pins that use the gitlab: backend.

Why

Renovate's built-in mise manager maps the core, npm, aqua, pipx, github and cargo backends to datasources, but has no gitlab: backend handling (lib/modules/manager/mise/backends.ts). A gitlab:<group>/<project> pin is invisible to it: no error, no Dependency Dashboard entry, it simply never bumps.

That is the worst failure shape a toolchain pin can have, because the absence of an MR is indistinguishable from being up to date.

How

Wires those lines to the gitlab-releases datasource, with packageName captured from the path after the gitlab: prefix and semver-coerced versioning so a tag carrying a leading v still compares against a pin that does not. The built-in mise manager continues to handle every non-gitlab line in the same file, so this only fills the gap.

Verification

The pattern extracts all three gitlab: pins from a real toolchain baseline, among them gitlab-org/cli at 1.109.0. No consumer in this namespace carries a mise.toml today, so this is enabling rather than a fix to existing rot.