feat(config): configurable main branch

[rlodico]

Details

Enables the main branch name to be configurable so snapshots from branches other than master can be non-temporary.

Does this PR introduce a breaking change?

• Yes
• No

Note: I made master the default branch for seamless backwards compatibility, but we may eventually want to forgo that since the default branch on Github has changed from master to main. Thoughts on doing that now?

Another slightly more seamless transition might be to have mainBranch be nullable, and if undefined it accepts both master and main as non-temporary branches in store.ts#L86 (more complicated, but supports both):

    temporary: globalConfig.mainBranch
        ? (globalConfig.mainBranch !== branch)
        : ('master' !== branch && 'main' !== branch)