Rename the built-in BatchFileAdapter class back to FileAdapter

[hsluoyz]

See my comments here: #234 (comment)

I think first we need correctify the BatchFileAdapter code: rename BatchFileAdapter back to FileAdapter. From now on, FileAdapter will always be called FileAdapter, no matter how many interfaces it has implemented.

node-casbin/src/persist/batchFileAdapter.ts

Lines 8 to 28 in 27005f8

	export class BatchFileAdapter extends FileAdapter implements BatchAdapter {
	/**
	* FileAdapter is the constructor for FileAdapter.
	* @param {string} filePath filePath the path of the policy file.
	*/
	constructor(filePath: string) {
	super(filePath);
	}
	// addPolicies adds policy rules to the storage.
	// This is part of the Auto-Save feature.
	public async addPolicies(sec: string, ptype: string, rules: string[][]): Promise<void> {
	throw new Error('not implemented');
	}
	// removePolicies removes policy rules from the storage.
	// This is part of the Auto-Save feature.
	public async removePolicies(sec: string, ptype: string, rules: string[][]): Promise<void> {
	throw new Error('not implemented');
	}
	}

[Zxilly]

So we should move the batch functions and update function to FileAdapter?

[hsluoyz]

@Zxilly see Go code.

[nodece]

@hsluoyz @Zxilly I don't recommend that rename the BatchFileAdapter to FileAdapter, some users may be using this, so we can copy BatchFileAdapter code to FileAdapter, and add @deprecated to BatchFileAdapter.

[hsluoyz]

@nodece, @Zxilly, good idea, and it can be better:

We keep BatchFileAdapter but it's only an alias to FileAdapter, maybe by inheriting FileAdapter without any new functions. So we don't need to copy code. And we will add @deprecated to BatchFileAdapter. So we will not break backward compatibility.

[Zxilly]

@nodece @hsluoyz have implemented, plz review
915eff3