Feat: complete text ledger

[AlexTheWizardL]

This PR finalizes the TextLedger implementation by adding support for the following entities:

• Tax Codes
• Accounts
• Settings

In addition, a new file, decorators.py, has been introduced, containing the timed_cache() decorator. This decorator enhances the architecture by providing a more efficient caching mechanism with entity-specific cache timeouts.

Note: It is recommended to review this pull request commit by commit for better clarity.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.60140% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyledger/text_ledger.py	98.18%	2 Missing :warning:

Flag	Coverage Δ
unittests	77.55% <98.60%> (+2.86%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pyledger/constants.py	100.00% <100.00%> (ø)
pyledger/decorators.py	100.00% <100.00%> (ø)
pyledger/standalone_ledger.py	70.04% <100.00%> (+0.85%)	:arrow_up:
pyledger/text_ledger.py	95.21% <98.18%> (+7.33%)	:arrow_up:

[lasuk]

As a minor point, I suggest to reduce the cache timeout for TextLedger to 10, 15 or 30 seconds (pick your preference).

-> Reading information from file takes a tiny fraction of a second. Fresh file read every 10-30 seconds is absolutely acceptable. -> Reduces the time, during which a potential external change (i.e. file modification) is undetected.