chore: lock submodule dependencies to specific commit hashes

[AustinGreen]

Motivation:

Locking in submodule dependencies ensures that forge install will be reproducible.

Modifications:

Submodule commit hashes are added to .gitmodules. The commit hash of the most recent release was used for all dependencies.

Result:

Closes #15

[github-actions]

Coverage after merging austin/hardcode-submodules into main will be

97.40%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src/token-voting
ERC20TokenholderActionCreator.sol	87.50%	50%	100%	100%	37–38
ERC20TokenholderCaster.sol	92.31%	50%	100%	100%	39
ERC721TokenholderActionCreator.sol	84.21%	50%	100%	100%	37, 39–40
ERC721TokenholderCaster.sol	91.67%	50%	100%	100%	39
LlamaTokenVotingFactory.sol	100%	100%	100%	100%
TokenholderActionCreator.sol	90.91%	83.33%	83.33%	97.56%	117–118, 223–224
TokenholderCaster.sol	91.58%	86.76%	84.21%	96.12%	197–201, 281–282, 315–316, 363–364, 388–389