Fix inexact amounts in transction confirmation screen

[onyb]

Motivation: We want users to sign exactly what they enter in amount fields. For this to happen, we must display unaltered amounts in the transaction confirmation screen (panel).

Summary of changes:

• Refactored utils in format-prices.ts to use bignumber.js. Fiat values are always formatted to two places of decimals, unless less than 0.01. Crypto values are no longer formatted using the log stuff - my reasoning is that formatWithCommasAndDecimals is always passed values that are pre-formatted using utils in format-balances.ts, so we don't need another level of formatter. I still keep most of the logic as-is.
• Added a valueExact field to transaction parser response, to be used in the transaction confirmation screen.

Resolves brave/brave-browser#20432.
Resolves brave/brave-browser#19521.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Demo

Here are the transaction screens after creation a transaction with value 0.037314235731347379 ETH.

Before	After

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[muliswilliam]

LGTM