Introduce parse_number() helper function

[oxan]

What does this implement/fix?

This is the first in a series to clean-up helpers.h and helpers.cpp, and to provide consistently-named, generic helper functions. Final result will look something like this (but note that not everything in there is working/tested yet).

Introduce the parse_number() template function that properly parses a decimal number:

• Overloads for std::string and C-style strings
• Returns optional<T>
• Checks for over- and underflow
• Does not accept trailing garbage

Convert all uses of the existing parse_int() and parse_float() helpers, and drops them.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP32 IDF
• ESP8266

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

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